Knees and elbows!

Generally speaking, the same principles can be applied to knees and elbows. Below is an example of a knee joint.

In the illustration below, the green leg represents a poor geo layout, and the gold leg represents a good geo layout. What's the difference between the two? Notice the blue highlighted loops around the knees: the loop goes around the entire green leg, and on the gold leg is terminated before it wraps around to the back of the knee.

Most people structure their legs and arms as depicted on the green leg. However, when that joint bends it will not retain shape as well as the gold leg. Better deformations for less geometry?! It's true, here's why:

By giving that joint a horizontal edge to bend upon on the green leg, I have given it a place to collapse upon. In the illustration below, notice how the gold leg retains it's shape through flexion, while the green leg collapses along that central edge as it bends (hasn't even bent that far!).

These are very low poly examples, however the collapsing only increases as you add more and more polys into the equation. Better deformations for fewer polys- how economical!