A Tetris board exists on two levels: It is a frame, a container of graphical objects. It's where Tetris pieces show up. It also contains a grid- a data structure- of squares, each square capable of answering the question "are you occupied?" with either a boolean true or a boolean false. If a grid square is occupied, a piece's squares can't move into that grid square's position. The board can also remove a row of occupied squares.