One way to simulate depth space on the two dimensional plane is to place objects clearly behind / in front of each other. No sh*t? Yah, f'real! Placing things behind / in front of each other has a shorter name: overlap. Overlap is the 'atom' of depth space: learn how to manage it as it is very, very important. See below for a diagrammatic representation of overlap. You may regard depth space as being an advanced form of overlap.