Order of a projective plane

Summary

The order of a finite projective plane is the number $n$ such that every line has $n + 1$ points. It determines all the basic counts: each point is on $n + 1$ lines, and there are $n^2 + n + 1$ points and as many lines. The plane $PG(2, q)$ over the field with $q$ elements has order $q$, and which other orders occur is a central open question of finite geometry.