A pentomino is a plane geometric figure formed by joining five equal squares edge to edge. It is a polyomino with five cells. There are twelve pentominoes, not counting rotations and reflections as distinct. They are used chiefly in recreational mathematics for puzzles and problems. Pentominoes were formally defined by American professor Solomon W. Golomb starting in 1953 and later in his 1965 book Polyominoes: Puzzles, Patterns, Problems, and Packings. Golomb coined the term “pentomino” from the Ancient Greek πέντε / pénte, “five”, and the -omino of domino, fancifully interpreting the “d-” of “domino” as if it were a form of the Greek prefix “di-” (two). Golomb named the 12 free pentominoes after letters of the Latin alphabet that they resemble.
Ordinarily, the pentomino obtained by reflecting or rotating a pentomino does not count as a different pentomino. The F, L, N, P, Y, and Z pentominoes are chiral; adding their reflections (F’, J, N’, Q, Y’, S) brings the number of one-sided pentominoes to 18. Pentominoes I, T, U, V, W, and X, remain the same when reflected. This matters in some video games in which the pieces may not be reflected, such as Tetris imitations and Rampart.
John Horton Conway proposed an alternate labeling scheme for pentominoes, using O instead of I, Q instead of L, R instead of F, and S instead of N. The resemblance to the letters is more strained, especially for the O pentomino, but this scheme has the advantage of using 12 consecutive letters of the alphabet. It is used by convention in discussing Conway’s Game of Life, where, for example, one speaks of the R-pentomino instead of the F-pentomino.