Pentomino

A pentomino is a polyomino composed of five (Ancient Greek πέντε / pénte) congruent squares, connected along their edges (orthogonal connections). There are 12 different free pentominoes, named after the letters of the Latin alphabet they resemble.

Ordinarily, the pentomino obtained by reflection or rotation of a pentomino does not count as a different pentomino. The F, L, N, P, Y, and Z pentominoes are chiral in two dimensions; adding their reflections (F', J, N', Q, Y', S) brings the number of one-sided pentominoes to 18. The others, lettered I, T, U, V, W, and X, are equivalent to some rotation of their mirror images. This matters in some video games, where mirror image moves are not allowed, such as Tetris-clones and Rampart.

Each of the twelve pentominoes can be tiled to fill the plane. In addition, each chiral pentomino can be tiled without using its reflection.

John Horton Conway proposed an alternate labeling scheme. He uses O instead of I, Q instead of L, R instead of F, and S instead of N. The resemblance to the letters is a bit more strained (most notably that the "O," a straight line, bears no resemblance to an actual letter O), but this scheme has the advantage that it uses 12 consecutive letters of the alphabet. This scheme is used in connection with Conway's Game of Life, so it talks about the R-pentomino instead of the F-pentomino.