A partition of is a disjoint collection of sets whose union is ; often these subsets are called blocks. Two blocks and are crossing if they contain elements and such that . (These situations are not crossing: , .) A noncrossing partition is a partition with no crossing pair of blocks.

This Demonstration shows two figures related to noncrossing partitions: one in which points are arranged at the corners of a regular polygon, with line segments connecting members of the same block; and one in which points are arranged in a line, with arcs joining members of the same block.