A polyonmino is the name given to plane figures created by groups of squares
touching at their edges. Polyominoes are generally referred to in groups, sharing a
characteristic number of sides, ignoring rotations and reflections. For example, the
monomino is the trival group consisting of a single square. Nearly everyone is familiar with
the shape of the domino, a rectangle consisting of two side-by-side squares. The trominoes
are slightly more interesting: this group has two members, referred to as the "straight
tromino" and the "right (angle) tromino".
Polyominoes made of four squares are referred to as tetrominoes, and there are five
of these. The popular arcade puzzle game "Tetris" challenges players to interlock these
polyominoes while leaving as few holes as possible.
The five-square polyominoes are called the pentominoes. Twelve distinct pentominoes
exist. For convenience, each can be thought as resembling a letter of the alphabet and hence
is given a "letter name".