Fingers!

Fingers are very similar to knees and elbows in regards to how they hinge in one direction, so the core structure around the joints is essentially the same.

Fingers go through a lot of stress when animated! Something as common as a hand making a fist can look horrible if the fingers aren't structured correctly. When done right, there is no crushing and fewer lighting anomalies. When done wrong, there is a ton of crushing (more joints to bend!) and a ton of lighting inconsistencies across the model.

The below illustration represents two fingers at a base geometry level. The green is once again representing a bad layout, and the gold a good one. The loops in question are the ones highlighted in blue, bisecting the knuckles.

By giving the knuckles a horizontal edge to bend upon on the green finger, I have given it a place to collapse upon. In the illustration below, notice how the gold finger retains it's shape through flexion, while the green finger collapses along that central edge as it bends.

The geo theory behind these hinge joints was originally taught to me by Chris George, a handfull of years ago when we both worked at Left Field Productions. Check out Chris' webpage at www.superostrich.net