The remainder of Part One discusses each of the six perspective techniques: overlap; size and space variation; modeling; detail and edges; color and value change; converging lines. Let's begin with overlap.
Suppose you had to bet your last dollar on which object in the sketch at top right is farthest forward. Could you win? There's really no way to know for sure without receiving a clue. The box and ball could be sitting side by side, or the box could be a building so far off in the distance that it looks nearly as small as the ball.
The clue can be provided by simply overlapping the two objects. In the second sketch I've put the ball ...