Hi there,
This 'problem' torments me since a while:
Let us draw one projection of a square, and the projection is seen as a rhomboid - so, draw some arbitrary rhomboid. (Imagine a square tile that you see from a certain angle - it will become slanted and therefore distorted and seen as a series of romboids as you change the angle of view. Have in mind that distance at/from which you see it does not matter - only the angle.)
Q: How to find the 2nd projection of this square, and how to draw this square (full size, seen 'en face', as if seen directly under 90°)? See the image below.
Milos
P. S. You can rotate the image by 90 deg or change the shape of the rhomboid as well, the case is meant to be general and thereby doesn't change the essence.