This lesson is dedicated to the perspective of the points that belong to line **r **and, particularly, to some of the points that are notable because of their position.

The perspective of a point **P**, that belongs to line **r**, is the intersection of the projecting line (**OP**) with the perspective **r’** of the line **r**.

Obviously, any point **P** that belongs to the straight line **r** has a perspective **P’** that belongs to the perspective **r’** of the line. But it is also true that any point **Q’** of **r’** is the perspective of a point **Q** that belongs to line **r**. We say that this correspondence is bi-univocal.