Greatest element and least element
Top and bottom

The least and greatest element of the whole partially ordered set play a special role and are also called bottom (⊥) and top (⊤), or zero (0) and unit (1), respectively.If both exist, the poset is called a bounded poset.The notation of 0 and 1 is used preferably when the poset is a complemented lattice, and when no confusion is likely, i.e. when one is not talking about partial orders of numbers that already contain elements 0 and 1 different from bottom and top.The existence of least and greatest elements is a special completeness property of a partial order.

Further introductory information is found in the article on order theory.
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