First post on this topic here.
CURVES NEVER HAVE THEIR HIGH POINT IN THE CENTER. If you are drawing a curve (say, along the edge of a shoulder), the high point of the curve will never be at the center of the curve. It will always be closer to one end point than another.
FORMS DO NOT MIRROR EACH OTHER. Think, for example, of a forearm. The curve of the outside of the forearm will never be a mirror image of the inside curve. Throughout the whole body, inside and outside curves are never the same. When you find such a mirrored curve in a drawing or painting, it will almost always be inaccurate. The upper eyelid is not a mirror of the lower eyelid. The left side of the calf is not a mirror of the right side. The shapes of the curves will be different; their lengths, amplitudes, and high points will be in different places. The only exception is this: because the body is approximately symmetrical from left to right, you can sometimes find a form on the right side of the body that comes close to mirroring a form on the left side. But because we don’t normally stand or sit exactly vertically, it almost never works out that way, either.
I was looking at a magazine for artists the other day, and there was an article on the work of an instructor in figurative are. He runs a school to teach others all about academic figure drawing. But he mirrors his forms all the time. His work looks rather clumsy as a result.