Tetris Game Online: Play Tetris!. The player cannot prevent this fall, but can decide the rotation of the piece and where it should fall. When a horizontal line is completed, that line disappears and all the pieces that are above it descend one position, freeing up playing space and therefore facilitating the task of placing new pieces. It is made up of different "tetriminos", geometric figures made up of four square blocks joined together orthogonally.