Imaginary Underwear

It's true that knowing the nude helps in drawing clothing, but sometimes, knowing clothing helps in drawing the nude.  Introducing : Imaginary Underwear!

Brassiers fasten beneath the shoulder blades, and wrap around the ribcage to support the bosom.
Panties ride on the hips, filling a gap beneath the abdomen.  When removed, we can see a 'ghost outline' of the space this garment was designed to fit.

Drawing "imaginary underwear" onto the nude helps locate important anatomical landmarks, and creates surface lines which aid in understanding the body in three dimensions.