Linear perspective is a depth cue that is related to both relative size and the next depth cue, texture gradient. In linear perspective parallel lines that recede into the distance appear to get closer together or converge.
Here
is Paris Street: A Rainy Day by Gustave Caillebote. Look at the building
in the background. The separation between the floors are clearly indicated.
No, these lines should be parallel or people would have very odd experiences
walking from one end of the floor to the next. Yet, these lines converge
and give the indication that the building is angled and that the surfaces
recede in depth. Thus, parallel lines indicate a flat surface and converging
lines that we see as parallel indicate a surface that recedes in depth.
Information provided by: http://krantzj.hanover.edu