What does a 4-element tuple argument for ‘bbox_to_anchor’ mean in matplotlib?

In the “Legend location” section of the “Legend guide” in the matplotlib website, there’s a small script where line 9 is plt.legend(bbox_to_anchor=(0., 1.02, 1., .102), loc=3, ncol=2, mode="expand", borderaxespad=0.). All the tuples I’ve seen passed to bbox_to_anchor have 2 elements in it, but this one has 4. What does each element mean if the tuple passed has 4 elements?