The first moves of a backgammon game are the opening moves, collectively referred to as the opening, and studied in the backgammon opening theory. Backgammon opening theory is not developed in as much detail as opening theory in chess, which has been widely studied. The reason for this is that following the first move in backgammon, there are 21 dice roll outcomes on each subsequent move, and many alternative plays for each outcome, making the tree of possible positions in backgammon expand much more rapidly than in chess.