A random sequence of tetrominoes (sometimes called "tetrads" in older versions)
- shapes composed of four square blocks each - fall down the playing field. The
object of the game is to manipulate these tetrominoes with the aim of creating a
horizontal line of blocks without gaps. When such a line is created, it disappears,
and the blocks above (if any) fall. As the game progresses, the tetrominoes fall
faster, and the game ends when the player "tops out", that is, when the stack of
tetrominoes reaches the top of the playing field and no new tetrominoes are able
to enter. (The exact definition of a top-out varies from version to version.)
The seven one-sided tetrominoes in Tetris are referred to as I,
J, L, O, S, T, and Z. All are capable
of single and double clears. I, J, and L are able to clear
triples. Only the I tetromino has the capacity to clear four lines simultaneously,
and this clear is referred to as a "tetris." [2] (This may vary depending
on the rotation and compensation rules of each specific Tetris implementation.
For instance, in the Tetris Worlds type rules (see below) used in many recent
implementations, certain rare situations allow T,S and Z to
`snap` into tight spots, clearing triples.)
The seven possible Tetris pieces in their Tetris Worlds colors.
Top row: I, J, L, O. Bottom row: S, T, Z.
