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, 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.)