viewed from an oblique angle, or squashed (scaled on one axis), becomes an ellipse:
An ellipse is symmetrical on both center lines - all four quadrants are the same shape (some are mirror versions of the others). The long center line of an ellipse is called the "major axis", the short center line is the "minor axis."
But shapes in perspective are not symmetrical - they get smaller as they recede into space, right? For example, a square in perspective looks like this:
So the top "half" of the perspective circle is a very different size and shape from the bottom half:
Go to Part Two