Front Corners & Back Corners
Imagine that the sitting surface of your seat ( minus the thighs ) were in the shape of a rectangle. You would have front corners and back corners. Ideally you should have 50/50 weight to front and back. Where does your weight go? In the diagrams attached you can see how different postures would place the weight in different places. Hollow backed riders would have more weight in their front corners, and round backed riders would have more weight in their back corners.
A more advanced question would be, are the left and right corners of your front (and back) equal?
The first drawing is the most correct, with the rider in neutral spine, which will show slight curve in the lower back.
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