Each Tetris piece is made up of four blocks.
These four square blocks form a tetromino, a shape whose name comes from “tetra,” meaning four. In standard Tetris, every falling piece is one of seven tetromino shapes: I, O, T, S, Z, J, or L. The pieces differ in arrangement, but not in the number of blocks they contain.
The four-block design was a deliberate simplification by Tetris creator Alexey Pajitnov. He had been inspired by pentomino puzzles, whose pieces contain five squares and have twelve basic shapes. Pajitnov found that using four squares produced fewer shapes—seven in the one-sided set—making the game easier to learn while preserving plenty of strategic variety.
Players often describe these units as squares or blocks, but “tetromino” is the more precise term for the complete piece. The individual cells can be moved and rotated as a unit, and when placed they become part of the stack. Completing a horizontal line with these blocks causes that line to disappear.