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A tetromino, also spelled tetramino or tetrimino, is a geometric shape composed of four squares, connected orthogonally. This is a particular type of polyomino, like dominoes and pentominoes are. Sometimes the term is generalized to apply to configurations of four orthogonally connected cubes.

A popular use of tetrominoes is in the video game Tetris.

Counting rotations in two dimensions as equivalent, there are seven possible shapes:

When added the third dimension, there are three more patterns, all created by placing a unit cube on the bent trimino:

However, going to 3 dimensions means that rotation is allowed in 3 dimensions. Thus, the two L-shaped and S-shaped pieces are now equivalent.

Some people refer to the pieces by the colour in which they are drawn in a particular implementation of the Tetris game, but those colours vary from implementation to implementation so this is not very sensible. For example, in many older versions of Tetris, the red piece is I.
Colors of pieces in various implementations (under construction)
PieceVadim Gerasimov's
original Tetris
Microsoft TetrisThe New Tetris
Note that the box art of The New Tetris reverses the colors of the L and J pieces and the S and Z pieces.

See also: Domino, Pentomino, Polyomino