Points, lines and planes are the bricks that allow building a geometric Model of the World.
A point is the entity that two incident lines have in common.
A line is the entity that two incident surfaces have in common.
Particularly, a straight line is the entity that two incident planes have in common.
The projection is the fundamental operation that allows you to generate an image of the Model and to control, by means of this image, any geometrical features of the Model.
The projection needs a point, which will be assumed as Centre of projection, and a surface (normally a plane π), which will be assumed as Picture (normally called Picture Plane).
In these lessons, and unless otherwise noted, the picture plane is always vertical. and therefore perpendicular to the horizontal reference plane.