Four squares, joined edge to edge
A tetromino is a shape made by joining four equal squares along their edges. The name belongs to a larger mathematical family: a monomino has one square, a domino has two, a tromino has three, and a tetromino has four. The pieces may be turned, but squares that touch only at a corner do not count as joined.
That definition is more precise than “block.” It gives designers a small, orderly set of shapes to work with. In Tetris, the familiar seven pieces are the possible four-square forms when rotations count as the same shape, but mirror images remain distinct. That distinction accounts for pairs such as the left- and right-facing L shapes.
Why the geometry matters in play
Each piece occupies four cells, so fitting one into a gap is a problem of matching its outline to the board. Rotating changes where those four cells land; it does not change the piece’s identity or the space it ultimately fills. When a row becomes completely occupied, it can be cleared, creating room for the next pieces.
The formal name reflects the game’s underlying constraint: every falling shape is built from the same number of squares, yet their different arrangements create very different openings and dead ends. A tetromino is not just a block to drop—it is a four-cell puzzle whose shape determines what the board can accept.
