Hasse diagram
In order theory, a Hasse diagram (; German: [ˈhasə]) is a type of mathematical diagram used to represent a finite partially ordered set, in the form of a drawing of its transitive reduction. Concretely, for a partially ordered set ( S , ≤ ) {\displaystyle (S,\leq )} one represents each element of S {\displaystyle S} as a vertex in the plane and draws a line segment or curve that goes upward from one vertex x {\displaystyle x} to another vertex y {\displaystyle y} whenever y {\displaystyle y} covers x {\displaystyle x} (that is, whenever x ≠ y {\displaystyle x\neq y} , x ≤ y {\displaystyle x\leq y} and there is no z {\displaystyle z} distinct from x {\displaystyle x} and y {\displaystyle y} with x ≤ z ≤ y {\displaystyle x\leq z\leq y} ).