How does overlap suggest front and back?

When one book partly hides another, the hidden book does not stop at the visible edge. We understand that some of it continues behind the nearer book. A flat drawing can use this same clue: one shape covers part of another shape's outline, suggesting front and back.

This clue is called overlap or occlusion. Here the drawn objects represent opaque, unbroken layers, so we cannot see through them. If an edge disappears where another shape covers it, the covering shape is in front at that location. The picture suggests depth even though the page itself is flat.

In the example, circle B covers part of rectangle A, and triangle C covers part of B. Under this consistent layer model, C is in front of B, which is in front of A. The missing edge portions provide the evidence. No lines need to meet at a distant vanishing point.

Overlap does not always establish every relationship. If C covers both A and B but A and B never overlap, C is in front of both while A's order relative to B remains unknown. Nor does an overlap tell us how many centimeters separate the objects. It supplies a useful depth relationship without providing a complete three-dimensional measurement.