Tetris (Russian: Тетрис) is a falling-blocks puzzle video game, released on a vast spectrum of platforms. Alexey Pajitnov originally designed and programmed the game in June 1985, while working for the Dorodnicyn Computing Centre of the Academy of Science of the USSR in Moscow. Pajitnov has cited pentominoes as a source of inspiration for the game. He derived its name from the Greek numerical prefix "tetra-", as all of the pieces contain four segments, and tennis, Pajitnov's favorite sport.
The game (or one of its many variants) is available for nearly every video game console and computer operating system, as well as on devices such as graphing calculators, mobile phones, portable media players, and PDAs.
A pseudorandom 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, by moving each one sideways and rotating it by 90 degree units, 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).
Tetris game manuals refer to the seven one-sided tetrominoes in Tetris as I, J, L, O, S, T, and Z - due to their resembling letters of the alphabet - but players sometimes use other names for the pieces. 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 is referred to as a "tetris" (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).