understanding Debian dpkg package version numbers

When I type dpkg -l | grep xserver-xorg, I get list of installed packages and their versions:

ii  xserver-xorg               1:7.7+3~deb7u1
ii  xserver-xorg-core          2:1.12.4-6
ii  xserver-xorg-input-evdev   1:2.7.0-1+b1
ii  xserver-xorg-input-kbd     1:1.6.1-1+b1
ii  xserver-xorg-input-mouse   1:1.7.2-3
ii  xserver-xorg-video-radeon  1:6.14.4-8

I am wondering what the number infront of the colon means, ie the number 2 in 2:1.12.4-6

Answers:

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Method 1

The version numbers in Debian are of the form

[epoch:]upstream_version[-debian_revision]

where

• epoch is a single (generally small) unsigned integer, which is included to allow mistakes in the version numbers of older versions of a package. If omitted, the epoch is assumed to be zero.
• upstream_version is usually the version number of the original source package from which the .deb file has been made. It is usually kept the same as the format used for the upstream source.
• debian_revision specifies the version of the Debian package based on the upstream version. It is optional and is omitted in cases where a piece of software was written specifically to be a Debian package.

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