What are the key principles that effective designs share?

You have arrived on this page because you are clearly curious about underlying fundamentals that you can incorporate into your thought process when designing.

According to many that came before us, there are key principles that can be considered when creating an effective design. In the following answers, you will find useful theories from a variety of respected sources.

Use this question as a reference for navigating the variety of topics:

Gestalt Principles

Other Fundamentals and Useful Information

If you know of a topic that hasn’t been covered here, please add an answer with references to respected resources and it will be added to the list.

Of course direct quotes and paraphrasing from respected resources are a must, but should be limited to a small paragraph per topic at most and referenced back to their source as thoroughly as possible.

A question very similar in nature has been posted before, but not to create a collection of snippets that are enlightening for graphic designers of any and all skill levels.

Method 1

The key principles of effective design

Communication is above all the forefront of effective design. Graphic Design by nature is about communicating an idea. Regardless of medium this is the de facto standard for an effective design.

Quick Historical Look:

Nearly 200 years ago William Playfair (1786) began the serious use of graphs for looking at data. More than 50 years ago a battle raged on the pages of the Journal of the American Statistical Association about the relative merits of bar charts and pie charts (Eells 1926; Croxton 1927; von Huhn 1927). Today graphs are a vital part of statistical data analysis and a vital part of communication in science and technology, business, education, and the mass media.

Source: Graphical perception: Theory, experimentation, and application to the development of graphical methods in the Journal of the American Statistical Association, Vol. 79, No. 387. (Sep., 1984), pp. 531-554.

Larkin and Simon (1987) suggest that the following forms of cognitive efficiency are offered by good graphical displays.

• Substituting Visual Operators: Graphical displays often allow users to substitute less demanding visual operators in place of more complex logical operators in place of more complex logical operators. Visual operators (e.g., distance and color comparisons, spatial coincidence judgements) can often give users the same information as more complex non-visual operators.
• Reducing Search: Graphical displays often arrange information in such a way as to reduce the number of items the user must look at in order to find something useful, or to group information required to draw a particular inference into one spatial locality. Graphical techniques like shading and spatial arrangement can help guide the eye to relevant information or past irrelevant information.

A lot of designers especially today get overshadowed by creativity and can get too far from the initial goal of communicating a goal. Here’s a passage from Visual Design Principles for Usable Interfaces (Watzman, Suzanne) that I think is very relevant:

Defining Visual Design. Visual design is not merely a series of subjective choices based on favorite colors or trendy typefaces–at best a cosmetic afterthought considered if there is enough time and money. Good visual design is the tangible representation of product goals. It is concerned with the “look,” the method, and the style in which the information is presented. It should be the result of a thoughtful, well-considered process, not merely a decorative afterthought.

Applying the appropriate visual/experience design principles and tools while incorporating the user perspective (information design) enhances the value, perception, and usefulness of products. It is the best combination of project goals, the user perspective, and informed decision making.

Design is in direct relationship with the economy. When we design for Engineering it is very easy to see how the design solution must meet certain economic goals. In architecture and graphic design this is often not the case. They can become ornate and saturated, especially when the economy is good. The budget is there. When the economy is bad then things get simplified, mass produced, reusable, templates and frameworks — all of it. There are design solutions that are possible within these parameters and they reduce the overall cost of goods. When times are good, or you get lucky enough to land a client with the money to spend then its easier to justify new paradigms and reasons.

Reducing Search: Where the Principles of Design come in

If the goal is to communicate an idea then to best do this, we as designers, must have some clue how people (unless you’re designing for other species) perceive the world around us.

Top-Down Processing. Widely seen as the bulk of our processing capabilities is that we start at the larger entities before working our way down into the details. In this way our expectations affect our perceptions. For examle you probably filled in the letter P just now, because your brain expected it to be there. There are a number of tests you can look at including, The Hollow Head, which tricks your brain into seeing what isn’t there. It does this because of top-down processing giving us expectations about what SHOULD be there, rather than what IS there. (An immediate example of this in the traditional realm of Graphic Design would be the cover of Invisible Monsters by Chuck Palahniuk:

Bottom-up Processing. Also called data-driven processing, is when you have no expectations. You have to start small and work to fill in holes until you find an expectation.

As a Graphic Designer you can think of this as the “aha” moment. If the majority of your intended audience can immediately identify the whole then they use almost all Top-Down Processing. This is a key ingredient for a successful design. It requires less searching from one spot to another in the hopes of figuring out what is going on.

I’m going to end this here, at least for now. At this point you can get into the more technical stuff like the Gestalt principles that Dominic has begun to discuss.

Application / Implementation

I realize some of this stuff seems fairly complex, particularly for new designers to grasp. Over the next few days I’m going to add images in this section but wanted to at least get some basic stuff in writing to help people out.

On your first iteration of an advertisement pretend you put in a picture of your product in use. Pretend for now its a water bottle being drunk by someone on a crowded basketball court.

You look at it and think, wow this is a great shot of our product! You show your coworkers and they think, wow our designer did a great job showing our product! You show the rest of the world though and you know what they’re going to think? What’s the product?

They’ll have to search the page for clues.

Second Iteration

To begin fixing this you might think, how can I make this clear. People should be able to quickly scan the ad and know exactly what it means. It should be as much top-down processing as possible.

So perhaps the first step is adding a logo. Put a logo on the bottom right and the logo is visible on the water bottle. That’s a good clue. Is it enough though? Probably not.

Third Iteration

You work on the copy and add some large text, “Always cold water! The world’s first water bottle with solar panels that can keep you hydrated on the hottest days!”

Alright, we’re making progress. Now someone sees a photo, identifies the logo, and discovers from the copy that we’re talking about some sort of water bottle.

Fourth Iteration

Circle the water bottle. Darken everything but the water bottle. Oversaturate it. Do something, anything, to make it pop! Now we’re doing graphic design. Now there’s almost no searching left. Without any text someone could look at your piece and immediately know to focus on the bottle. Maybe you differentiate the entire person with the bottle well thats okay, you still have the text to further support it.

Final Iteration

The idea is finding a balance. How many iterations it takes is up to you. But with any design of anything you have to either really look at it as though its your first time or get others to test it for you. Once someone knows the product then they have expectations. They’re looking for your product already. You want to make your design communicate what that product is as well as you can allowing end-users to understand, and remember it, with as little bottom-up processing as possible. The less they have to scan before understanding, the better.

(I’ll redo this example with pictures over the next few days)

Here is a great paper on Top-Down Processing that Transitions into those very principles: What’s up in top-down processing? by Cavanagh, Patrick.

Here’s a wonderful article that came out in The Atlantic today (May 5, 2014) which stays on fairly abstract terms non-designers can understand but does clearly state: Things You Cannot Unsee (And What That Says About Your Brain)

In short: what you know influences what you see.

Method 2

In order to use the principles properly we first need to understand how users interact with designs, how they think and what are the basic patterns of users’ behavior.

Part 1

How do users think?

• Users appreciate quality and credibility.
• Users don’t read, they scan.
• Web users are impatient and insist on instant gratification.
• Users don’t make optimal choices.
• Users want to have control.

Don’t make users think

According to Krug’s first law of usability,

the design should be obvious and self-explanatory.

When you’re designing, your job is to get rid of the question marks — the decisions users need to make consciously, considering pros, cons and alternatives.

Don’t squander users’ patience

In every project when you are going to offer your user some service or tool, try to keep your user requirements minimal. The less action is required from users to test a service, the more likely a random user is to actually try it out.

Strive for simplicity

The “keep it simple”-principle (KIS) should be the primary goal of design. Strive for simplicity instead of complexity.

Don’t be afraid of the white space

Actually it’s really hard to overestimate the importance of white space. When a new user approaches a design layout, the first thing he/she tries to do is to scan product/page and divide the content area into digestible pieces of information.

Complex structures are harder to read, scan, analyze and work with.

If you have the choice between separating two design segments by a
visible line or by some whitespace, it’s usually better to use the
whitespace solution.

Hierarchical structures reduce complexity (Simon’s Law):

the better you manage to provide users with a sense of visual
hierarchy, the easier your content will be to perceive.

Communicate effectively with a “visible language”

In his papers on effective visual communication, Aaron Marcus states

three fundamental principles involved in the use of the so-called
“visible language” — the content users see on a screen.

• Organize: provide the user with a clear and consistent conceptual
structure. Consistency, screen layout, relationships and navigability
are important concepts of organization. The same conventions and
rules should be applied to all elements.
• Economize: do the most with the least amount of cues and visual
elements. Four major points to be considered: simplicity, clarity,
distinctiveness, and emphasis.

Simplicity includes only the elements that are most important for
communication. Clarity: all components should be designed so their
meaning is not ambiguous. Distinctiveness: the important properties
of the necessary elements should be distinguishable. Emphasis: the
most important elements should be easily perceived.

• Communicate: match the presentation to the capabilities of the user.
The user interface must keep in balance legibility, readability,
typography, symbolism, multiple views, and color or texture in order
to communicate successfully. Use max. 3 typefaces in a maximum of 3
point sizes — a maximum of 18 words or 50-80 characters per line of
text.

Conventions are our friends

Conventional design of elements doesn’t result in a boring product In fact, conventions are very useful as they reduce the learning curve, the need to figure out how things work.

With conventions you can gain users’ confidence, trust, reliability

Test early, test often

This so-called TETO-principle should be applied to design project as usability tests often provide crucial insights into significant problems and issues related to a given layout.

Test not too late, not too little and not for the wrong reasons.

In the latter case it’s necessary to understand that most design decisions are local; that means that you can’t universally answer whether some layout is better than the other one as you need to analyze it from a very specific point of view (considering requirements, stakeholders, budget etc.).

Some important points to keep in mind:

According to Steve Krug,

testing one user is 100% better than testing none and testing one user
early in the project is better than testing 50 near the end.

According to Boehm’s first law,

errors are most frequent during requirements and design activities and
are the more expensive the later they are removed.

Testing is an iterative process. That means that you design something, test it, fix it and then test it again. There might be problems which haven’t been found during the first round as users were practically blocked by other problems.

usability tests always produce useful results. Either you’ll be pointed to the problems you have or you’ll be pointed to the absence of major design flaws which is in both cases a useful insight for your project.

According to Weinberg’s law,

a developer is unsuited to test his or her code.

This holds for designers as well. After you’ve worked on a design for few weeks, you can’t observe it from a fresh perspective anymore. You know how it is built and therefore you know exactly how it works — you have the wisdom independent testers and user of your design wouldn’t have.

Method 3

According to The Non Designers Design Book by Robin Williams, one of the four basic principles that apply to designing anything is Proximity.

Proximity

“Group related items together.”

When several items are in close proximity to each other, they become one visual unit rather than several specific units. Items relating to each other should be grouped together. The basic purpose of proximity is to organise.

Without considering the other principles, here is a simple example:

Disorganised list:

Desirable Community Member Traits
Happy
Fan of Cat GIFs
Enthusiastic
Mental in a good way
Necessary Community Member Traits
Respectful
Polite
Team Player
Community Focused

List organised with only the principle of Proximity applied:

Desirable Community Member Traits

Happy
Fan of Cat GIFs
Enthusiastic
Mental in a good way

Necessary Community Member Traits

Respectful
Polite
Team Player
Community Focused

As you can see, the principle of proximity is a basic but powerful fundamental for organising a design. Proximity is applicable to and relevant for all design elements, not just text.

One quick and simple way suggested in the book to evaluate the proximity relationships in your design is to “Squint your eyes slightly and count the number of visual elements on the page by counting the number of times your eye stops.”

Method 4

This question is fundamentally problematic. It seems an invitation get a laundry list of biases from people who have likely misunderstood design. Because the very phrasing:

What are the key principles that effective designs share?

…seems to be built on a notion that “graphic design” (a field of study and techniques) is anything more than heuristic for aiming at “effective design”. The most effective designs are matched to their audience in ways you can’t apply the comfortable heuristics to. To think otherwise is delusion.

If the question was “What are the key principles of graphic design?”… well, perhaps you could summarize the current state of the art. It’s a moving target; tastes change and so do perceptions. Not that a Q&A format is a good place for that summary, with upvotes and downvotes and comments and all of that.

I’ll try and write a 3-bullet point pretend answer to “key principles that effective designs share”:

• “An effective design would not lose information if it happened to be adjusted and seen in black and white; color should be an addition to a design, not the message. Because, um, I read that once and it seemed to have corrected a glaring mistake in an early design I made.”
• “If one decomposed an effective design mathematically into an equation, the number of parameters are optimized such as to reduce the total number of variants while still conveying the information. Thus, in essence…a good design is ‘compressed’. Hence all things being equal, you should snap near but not equal points to a -shared- grid point to avoid distractions created by small differences, which may distract the eye by seeming like a mistake. This is why every human uses graph paper (it’s the universal language).
• “An effective design doesn’t use Papyrus or Comic Sans. I learned this yesterday, but since I learned it yesterday, I now know tons more than those who might pick these from the combo box!”

I’ll restrain myself here. But so my so-far-unpopular answer is all I have:

The only principles that effective designs share is that they are effective and that they are designs.

As tautological as that may sound, this calls attention to the words themselves:

“Design”

It’s easiest for me to define design by what it isn’t. It stands in stark opposition to that accomplished by random exercises, such as simple juxtaposition.

In the absence of clear intent, the mind will search for meaning. It is trained to synthesize meanings on its own. Simple exercises can reveal the brain’s capacity for building its own interpretation. If you hand people a combination of a photograph and a phrase from newspapers or magazines… or ink blots, they go to town. But it’s their own town..

(The results may be fascinating, but it is far from calling the input to the process “design”.)

Design is communicable intent. It is the opposite of abstract art where viewers walk away, each with their own interpretation, and the artist smugly stands and says “it means whatever you want it to mean”. If everyone walks away with a different impression and they go “cool!” the work they saw was not the work of designers.

“Effective”

An effective design is applied toward a purpose. If the design does not meet the purpose, it is not effective.

The field of graphic design is a set of heuristics designed to assist those trying to solve design problems such that they do not have to independently survey each target audience each time they make something. If you print a red and green word in a 3 point font in the corner of a poster and overlap the two words, and then print another word in a 500 point bold font in white in the center, and no one sees or can read the little print but everyone notices the white print… that’s good. (If you wanted them to see the white print and not the overprinted stuff in the corner…)

Yet if you’re looking for people who are reading in the margins, and you want to hide, that is steganography. That’s a field of graphic design as well… just turned upside down. You are trying to filter against those who only see the rules. A design which is unseen by those you don’t want to see it (but seen by those you do) is by all definitions an effective design.

It might be in 3D, it might require glasses. It’s effective if you communicate. There is no unifying principle other than the tautological: it was designed, it is effective. Everything else is rehearsal of bias and not very interesting at the higher levels of design.

Thus I will say:

The core of every effective design, made by a designER, is the willingness to admit what they meant to accomplish, architecting the solution, and hitting the goddamn mark.

The end — no laundry list needed.

Method 5

Part 3

And I just can’t keep myself away from quoting my Favorite Dieter Rams 10 Principles of “Good Design”

Good Design Is Innovative :

The possibilities for innovation are not, by any means, exhausted. Technological development is always offering new opportunities for innovative design. But innovative design always develops in tandem with innovative technology, and can never be an end in itself.

Good Design Makes a Product Useful :

A product is bought to be used. It has to satisfy certain criteria, not only functional but also psychological and aesthetic. Good design emphasizes the usefulness of a product while disregarding anything that could possibly detract from it.

Good Design Is Aesthetic :

The aesthetic quality of a product is integral to its usefulness because products are used every day and have an effect on people and their well-being. Only well-executed objects can be beautiful.

Good Design Makes A Product Understandable :

It clarifies the product’s structure. Better still, it can make the product clearly express its function by making use of the user’s intuition. At best, it is self-explanatory.

Good Design Is Unobtrusive :

Products fulfilling a purpose are like tools. They are neither decorative objects nor works of art. Their design should therefore be both neutral and restrained, to leave room for the user’s self-expression.

Good Design Is Honest :

It does not make a product more innovative, powerful or valuable than it really is. It does not attempt to manipulate the consumer with promises that cannot be kept

Good Design Is Long-lasting :

It avoids being fashionable and therefore never appears antiquated. Unlike fashionable design, it lasts many years – even in today’s throwaway society.

Good Design Is Thorough Down to the Last Detail :

Nothing must be arbitrary or left to chance. Care and accuracy in the design process show respect towards the consumer.

Good Design Is Environmentally Friendly :

Design makes an important contribution to the preservation of the environment. It conserves resources and minimises physical and visual pollution throughout the lifecycle of the product.

Good Design Is as Little Design as Possible :

Less, but better – because it concentrates on the essential aspects, and the products are not burdened with non-essentials. Back to purity, back to simplicity.

My Ideology

• Content Comes First

Don’t put the cart before the horse. Define your content before you
decide to design

Consider the needs and expectations of your ideal user and appeal to
those users on a more intuitive level.

• Use appropriate media

Use the most relevant and user-friendly media availbale to convey
information

• Be intuitive

A clear structure, visual clues and easily recognizable design help
users to find the path to their aim.

• Don’t Fear Whitespace

Whitespace reduces cognitive overload and makes it easier to digest
the information presented on screen

• Remove Barriers

forcing them into sharing private data first

• Strive for Simplicity

Users rarely visit a site just to enjoy a design- they are looking for
information

• Don’t Reinvent the Wheel

Standard design conventions reduce the need for your users to figure

• Be Consistent

Keep the design and branding consistent everywhere.

• Call to Action

Guide and encourage users to perform useful actions and increase
usage.

References : (for all parts)

• Designing Effective User Interfaces by Suzanne Martin
• Summary on Web Design
• UID presentation (Flash)
• Research-Based Web Design & Usability Guidelines
• “The psychology of computer programming” by Gerald Weinberg
• “Designing Web Usability” by Jakob Nielsen [JN / DWU]
• “Prioritizing Web Usability” by Jakob Nielsen
• “Don’t Make Me Think” by Steve Krug
• “Usability for the Web: Designing Web Sites that Work” by Tom Brinck,
Darren Gergle, Scott Wood
• A Summary of Principles for User-Interface Design

Method 6

Part 2

The 7 Principles of Universal Design were developed in 1997 by a working group of architects, product designers, engineers and environmental design researchers, led by the late Ronald Mace in the North Carolina State University.The purpose of the Principles is to guide the design of environments, products and communications.

“may be applied to evaluate existing designs, guide the design process
and educate both designers and consumers about the characteristics of
more usable products and environments.”

Principle 1: Equitable Use

The design is useful and marketable to people with diverse abilities.

1. Provide the same means of use for all users: identical whenever possible; equivalent when not.
2. Avoid segregating or stigmatizing any users
3. Provisions for privacy, security, and safety should be equally available to all users.
4. Make the design appealing to all users.

Principle 2: Flexibility in Use

The design accommodates a wide range of individual preferences and abilities.

1. Provide choice in methods of use.
2. Accommodate right- or left-handed access and use.
3. Facilitate the user’s accuracy and precision.
4. Provide adaptability to the user’s pace.

Principle 3: Simple and Intuitive Use

Use of the design is easy to understand, regardless of the user’s experience, knowledge, language skills, or current concentration level.

1. Eliminate unnecessary complexity.
2. Be consistent with user expectations and intuition.
3. Accommodate a wide range of literacy and language skills.
4. Arrange information consistent with its importance.
5. Provide effective prompting and feedback during and after task
completion.

Principle 4: Perceptible Information

The design communicates necessary information effectively to the user, regardless of ambient conditions or the user’s sensory abilities.

1. Use different modes (pictorial, verbal, tactile) for redundant
presentation of essential information.
2. Provide adequate contrast between essential information and its
surroundings.
3. Maximize “legibility” of essential information.
4. Differentiate elements in ways that can be described (i.e., make it
easy to give instructions or directions).
5. Provide compatibility with a variety of techniques or devices used
by people with sensory limitations.

Principle 5: Tolerance for Error

The design minimizes hazards and the adverse consequences of accidental or unintended actions.

1. Arrange elements to minimize hazards and errors: most used elements,
most accessible; hazardous elements eliminated, isolated, or
shielded.
2. Provide warnings of hazards and errors.
3. Provide fail safe features.
4. Discourage unconscious action in tasks that require vigilance.

Principle 6: Low Physical Effort

The design can be used efficiently and comfortably and with a minimum of fatigue.

1. Allow user to maintain a neutral body position.
2. Use reasonable operating forces.
3. Minimize repetitive actions.
4. Minimize sustained physical effort.

Principle 7: Size and Space for Approach and Use

Appropriate size and space is provided for approach, reach, manipulation, and use regardless of user’s body size, posture, or mobility.

1. Provide a clear line of sight to important elements for any seated
or standing user.
2. Make reach to all components comfortable for any seated or standing
user.
3. Accommodate variations in hand and grip size.
4. Provide adequate space for the use of assistive devices or personal
assistance.

Method 7

According to The Non Designers Design Book by Robin Williams, one of the four basic principles that apply to designing anything is Contrast.

Contrast

“If two items are not exactly the same, then make them different. Really different.”

Contrast on a page draws our eyes to it; for contrast to be effective, the two elements must be very different. Contrast has two purposes, one is to create an interest on the page, the other is to aid in the organisation of the information.

Without considering the other principles, here is a simple example:

Disorganised list:

Desirable Community Member Traits
Happy
Fan of Cat GIFs
Enthusiastic
Mental in a good way
Necessary Community Member Traits
Respectful
Polite
Team Player
Community Focused

List organised with only the principle of Contrast applied:

Desirable Community Member Traits
Happy
Fan of Cat GIFs
Enthusiastic
Mental in a good way
Necessary Community Member Traits
Respectful
Polite
Team Player
Community Focused

Contrast alone on this example, is a very weakly effective way of organising the information. But as stated above, contrast is better utilised in drawing eyes towards it as a visual interest.

This does not mean make sure everything contrasts against everything else, but rather, use contrast to lead your audience around your design.

Add contrast through your typeface choice, line widths, colours, shapes, sizes, space, etc.

Don’t be a wimp. If you’re going to contrast, do it with strength.

Method 8

According to The Non Designers Design Book by Robin Williams, one of the four basic principles that apply to designing anything is Alignment. This is also one of the 125 universal principles covered in Universal Principles of Design. Excerpts from each book will be preceded with NDDB and UPD respectively.

Alignment

Definition from Universal Principles of Design:

“The placement of elements such that edges line up along common rows or columns, or their bodies along a common center.”

Advice from The Non Designers Design Book:

“Nothing should be placed on the page arbitrarily. Every item should have a visual connection with something else on the page.”

NDDB: To make all the elements on the page appear to be unified, connected, and interrelated, there needs to be some visual tie between the separate elements.

UPD: Elements in a design should be aligned with one or more other elements. This creates a sense of unity and cohesion, which contributes to the design’s overall aesthetic and perceived stability.

NDDB: Be conscious of where you place elements. Always find something else on the page to align with, even if the two objects are physically far away from each other. Even if the separate elements are not physically close on a page, they can appear connected, related, unified with the other information simply by their placement.

UPD: As with all such principles of this type, there are exceptions (e.g., the misalignment of elements to attract attention or create tension). However, these exceptions are rare, and alignment should be considered the general rule.

For most designs, align elements into rows and columns, or along a centerline. When elements are not arranged in a row/column format, consider highlighting the alignment paths.

Use left- or right-aligned text to create the best alignment cues, and consider justified text for complex compositions. The invisible column created by left-aligned and right-aligned text blocks presents a clear, visual cue against which other elements of the design can be aligned.

NDDB: “No matter how chaotic a well-designed piece may initially appear, you can always find the alignments within.”

Here are some quickly Googled examples of good vs bad alignment, of course alignment isn’t the only factor at play in these examples.

Method 9

According to The Non Designers Design Book by Robin Williams, one of the four basic principles that apply to designing anything is Repetition. The Universal Principles of Design does not consider repetition a universal principle but it does cover the wider concept of consistency. Excerpts from each book will be preceded with NDDB and UPD respectively.

Repetition & Consistency

Definition of its relation to design from Universal Principles of Design:

“The Usability of a system is improved when similar parts are expressed in similar ways.”

Advice from The Non Designers Design Book:

“Repeat some aspect of the design throughout the entire piece.”

NDDB: To make all the elements on the page appear to be unified, connected, and interrelated, there needs to be some visual tie between the separate elements.

The purpose of repetition is to unify and to add visual interest. Think of repetition as being consistent, which I’m sure you do already. Then push the existing consistencies a little further – can you turn some of those consistent elements into part of the conscious graphic design? Then take a look at the possibility of adding elements whose sole purpose is to create a repetition.

At first, simply find existing repetitions and then strengthen them. As you get used to the idea and the look, start to create repetitions to enhance the design and the clarity of the information.

UPD: Consistency enables people to efficiently transfer knowledge to new contexts, learn new things quickly, and focus attention on the relevant aspects of a task. There are four kinds of consistency: aesthetic, functional, internal and external.

Aesthetic consistency refers to consistency of style and appearance (e.g., a company logo that uses a consistent font, colour, and graphic). Aesthetic consistency enhances recognition, communicates membership, and sets emotional expectations.

Functional consistency refers to consistency of meaning and action (e.g., a traffic light that shows a yellow light before going to red). Functional consistency improves usability and learnability by enabling people to leverage existing knowledge about how the design functions.

Internal consistency refers to consistency with other elements in the system (e.g., signs within a park are consistent with one another). Internal usability cultivates trust with people; it is an indicator that a system has been designed, and not cobbled together. Within any logical grouping elements should be aesthetically and functionally consistent with one another.

External consistency refers to consistency with other elements in the environment (e.g., emergency alarms are consistent across different systems in a control room). External consistency extends the benefit of internal consistency across multiple, independent systems. It is more difficult to achieve because rarely observe common design standards.

UPD: Use consistent approaches when possible, but do not compromise clarity or usability for consistency. In the words of Emerson “A foolish consistency is the hobgoblin of little minds…”