Projection plans
Descriptive Geometry is a dihedral system, that is, a system that uses two projection planes. 

One is vertical and is called the Front Projection Plane (FPP), or φo (fi zero); the other is horizontal and is called the Horizontal Projection Plane (HPP), or νo (Niu zero). 

These planes intersect on a straight line called the x-axis.

The x-axis divides the projection planes into semi plans: in the Frontal Projection Plane there is the Upper Front Semiplane (UFS) and the Lower Front Semiplane (LFS); in the Horizontal Projection Plane.

There is the Anterior Horizontal Semiplane (AHS) and the Posterior Horizontal Semiplane (PHS).

The projection plans divide the space into four portions, called dihedrals: I, II, III and
IV.

The projection plans in perspective:

This perspective shows the projection plans, the
semi plans, the x-axis and the dihedrals. Is this the system
basic used in Descriptive Geometry. Usually, it is represented in this position, assuming the observer located at the 1st dihedral, on the left.