Answer:
Diagonal lines as diminishing scale
An exemplification of diminishing scale, diagonal lines moving towards each other as they move up or down a painting or sketch give the illusion of depth. A real world example of this is a straight road that becomes skinnier as it approaches the distant horizon. Another example is standing at one end of an empty hallway and watching the lines where the wall and floor meet visually move towards each other as they move to the other side of the room.
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