Partial geometry
An incidence structure C = ( P , L , I ) {\displaystyle C=(P,L,I)} consists of a set P {\displaystyle P} of points, a set L {\displaystyle L} of lines, and an incidence relation, or set of flags, I ⊆ P × L {\displaystyle I\subseteq P\times L} ; a point p {\displaystyle p} is said to be incident with a line l {\displaystyle l} if ( p , l ) ∈ I {\displaystyle (p,l)\in I} . It is a (finite) partial geometry if there are integers s , t , α ≥ 1 {\displaystyle s,t,\alpha \geq 1} such that: For any pair of distinct points p {\displaystyle p} and q {\displaystyle q} , there is at most one line incident with both of them.