This marvelous study of a (human?) rump is by the eagle-eyed Tom Fluharty:
Fluharty takes nothing for granted about the human butt. There are no shortcuts here-- nothing uniform or symmetrical. From start to finish, this drawing is based on what he actually sees and not what we all assume we know. Note the variety of his line, his sharp use of shadows for accents, and the active, dynamic result he has achieved. He even indicates the stitching at the seams, not because he's one of those detail fetishists, but to add a little pepper to his drawing.
Next we have another unorthodox treatment of the folds and creases caused by the human butt:
This one, by Robert Fawcett, is powered by those strong diagonal slashes.
If you drew the seat of someone's pants without looking, you'd never imagine these folds. Fawcett was a master of finding and strengthening the geometric shapes in nature.
Here's a third example of a master draftsman (Albert Dorne) with a sharp, incisive treatment of the relationship between the human fundament and the cloth that covers it.
These three wonderful drawings all demonstrate the power of keen observation, hard work and great visual curiosity.
On the other hand, there are reference books that purport to explain how folds and creases work. Famed artist Burne Hogarth wrote a book entitled Dynamic Wrinkles and Drapery: Solutions For Drawing The Clothed Figure. It contains all kinds of drawings with little dotted lines and arrows demonstrating Hogarth's theories about kinetic forces and wrinkles. Here he shows us how he thinks cloth folds around our butts:
I've always been baffled by Hogarth's many fans. His drawing strikes me as decidedly third rate. (Anyone out there want to help me see what I'm missing?)
I think this drawing is based more on Hogarth's theories than on what he actually sees. There is more education in Fluharty's single drawing above than in an entire 142 page book on drawing wrinkles.