Piece

A piece(or more commonly used block) in Tetris and similar games is a geometric shape consisting of a set of squares that is moved as a unit. A square is the part of a piece that fills one unit of the playfield. In some games, all blocks that make up a piece are colored the same; in others, especially falling piece games other than Tetris and its clones, they are colored differently.

Polyominoes
A polyomino is a piece made of two or more square blocks, where all blocks are connected through full coincident edges (as if squares on graph paper).


 * A free polyomino is one that may be translated (moved about), rotated, or reflected.
 * A one-sided polyomino is one that may be translated or rotated.
 * A fixed polyomino can only be translated.

Polyominoes come in several sizes:
 * 2 blocks make a domino: featured in Dr. Mario and Puyo Pop; seen in Tetris Blast.
 * 3 blocks make a tromino (or trimino): featured in Columns (fixed I only) and Pac-Attack (L only); seen in Tetris Blast (I, L) and Puyo Pop Fever (L).
 * 4 blocks make a tetromino (or tetramino or tetrimino): featured in Tetris and its clones. Lumines uses only O tetrominoes.
 * 5 blocks make a pentomino: featured in Daedalian Opus; seen in Tetris 2  Bombliss.

Polyominoes are often named by a Latin letter that resembles the shape of the polyomino. For instance, an "I" has all its blocks in a straight line, and a "T" consists of an "I" tromino with one or more blocks in a straight line connected to one side of the center of the "I".

Hinged polyominoes
A hinged polyomino or polyplet is similar to a polyomino but also allows connections at a point. In games that use hinged polyominoes, a piece may break apart at the diagonal connections when it lands, and the fragments may or may not be controllable afterward.


 * 4 blocks: Hinged tetromino (featured in Tetris 2).

Other polyforms
Hextris and Bikaka are variants of Tetris that use 4-cell polyhexes, or pieces created from regular hexagons. Bubble Bath Babes uses polyhexes that may be reflected around the X or Y axis but not rotated.