Fabric is soft and will therefore form a soft curve at the area of its fold. Consider the image to your left. The two folds in the cloth has been shaded like a cylinder because of its curve. There is a crease in the fabric where it folds over on the top and you can see the transition from the light parts to the dark. The fabric that is underneath is much darker than the material on top, because it has little light.
There aren't a lot of generalizations about the clothing. It's something that you'll have to train your eyes to notice.