**Theory**

- Consider a straight line
**r**, belonging to the reference plane π, and perpendicular to the picture plane._{1} - Set up a straight line
**r°**, parallel to**r**, and passing through the centre of projection**O**(we call that condition of a line or a plane simply*projecting*; therefore, a projecting line is any line that passes through**O**, and so on). - The line
**r**meets the picture plane in**Tr**, called*trace*of the straight line. Since**r**belongs to the reference plane,**Tr**belongs to the trace**t**of the reference plane.

The line**r°**meets the picture plane in*I’r*, called*vanishing point*of the straight line. Since**r**belongs to the reference plane,**I’r**belongs to the vanishing line**i’**of the reference plane.

**Notes
**Since the straight line

**r**is perpendicular to the picture plane,

**I’r**coincides with the principal point

**O°**.

In general, if a straight line belongs to a plane, the trace of the line belongs to the trace of the plane; the vanishing point of the line belongs to the vanishing line of the same plane.

**Practice
**To draw the perspective of a line perpendicular to the picture plane:

- Set up the trace
**Tr**of the straight line**r**on the trace of the plane to which it belongs. - Remember that the vanishing line of any straight line perpendicular to the picture plane coincides with the principal point
**O°**. - Then, draw the perspective
**r’**(**Tr I’r**).