# Comparability graph

The undirected graph $(P,E)$ on a partially ordered set $(P,{\le})$ in which two points are adjacent if they are comparable; that is, $xy$ is an edge of the graph if and only if $x < y$ or $y < x$. Comparability graphs are characterised by the property that in any odd length closed path $x_1,\ldots,x_{2n+1}$ with $n \ge 2$ (so all $x_i,x_{i+1}$ are adjacent) there exists at least one "chord" $x_i,x_{i+2}$ (subscripts being taken in cyclic order).