FIGURE 1. Three basic examples of posets. (a) The integers ordered by the usual ≤ form a chain. The element 2 is drawn above 1 since 1 ≤ 2, and they are connected by a line because 2 covers 1 in the sense that there is no integer x between 2 and 1 such that 1 ≤ x ≤ 2. (b) The four card suits are incomparable under a wide variety of card game rules and we draw them side-by-side to express this. This configuration
is called an antichain. (c) The set of partitions of three elements a, b and c ordered by partition containment forms a more complex poset that exhibits both chain and antichain behavior. One chain consists of the elements a|b|c, a|bc, and abc since each successive partition contains the previous. The elements a|bc, b|ac, and c|ab form an antichain because none of these three partitions contains another.