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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:

• I (also called "stick", "straight"): four blocks in a straight line
• Square (also called "O", "package", "block"): four blocks in a 2x2 square
• T: a row of three blocks with one added below the center
A common move with the T piece is to spin it in place to fill a line.
• L: a row of three blocks with one added below the left side
• J (also called "inverted L", "Gamma"): a row of three blocks with one added below the right side
This piece is a reflection of L but cannot be rotated into L in two dimensions; this is an example of chirality. However, in three dimensions, this piece is identical to L.)
• S: bent trimino with block placed on outside of clockwise side
• Z: bent trimino with block added on outside of anticlockwise side
Reflection to S. See "J", above.

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

• Left screw: unit cube placed on top of anticlockwise side. Chiral in 3D.
• Right screw: unit cube placed on top of clockwise side. Chiral in 3D.
• Branch: unit cube placed on bend. Not chiral in 3D.
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)
original Tetris
Microsoft TetrisThe New Tetris
Iredredcyan
Squarebluecyanwhite
Tbrowngrayyellow
Lmagentayellowmagenta
Jwhitemagentablue
Sgreenbluegreen
Zcyangreenred
Note that the box art of The New Tetris reverses the colors of the L and J pieces and the S and Z pieces.