# Closed subset of projective plane

From Groupprops

This term is related to: incidence geometry

View other terms related to incidence geometry | View facts related to incidence geometry

Template:Projective plane-subset property

## Definition

A subset of (the point-and-line-set of) a projective plane is said to be **closed** if it satisfies the following:

- Given any two points in the subset, the line incident with them is also in the subset
- Given any two lines in the subset, the point incident with them is also in the subset