We consider the following situations:

a) coincident or equal plans

b) competing or drying plans

Two planes, , are concurrent when their intersection is a single straight line:

c) parallel planes

Two planes, , are parallel when their intersection is empty:

## Perpendicularism between planes

Two planes, , are perpendicular if, and only if, there is a line of one that is perpendicular to the other:

Note: There are infinite planes perpendicular to a given plane; these planes may be parallel to each other or secant.

Next: Orthogonal Projection