In
backgammon, a prime is a block of six
contiguous points that have two or more of one's pieces on them, thereby blocking any of the opposing player's pieces behind the prime from crossing them and escaping. Primes are effective because in backgammon one moves pieces a
die at a time, ie. four then three, not seven. Since no die can have more than six on it, no piece can be moved more than six points at once and a prime of six is thus a total block until it's dismantled.
A prime on all six points of your inner board (the quarter of the board where your opponent's pieces start) is an inside prime; one with any of its points outside one's inner board is an outside prime, and one of less than six contiguous points is a partial prime. If you can put together an inside prime you will often win the game by a large margin, because if you then hit one or more of your opponent's blots he will be completely unable to reenter the board with that piece and must wait until you break the prime, which you ordinarily do only when bearing off at the end of the game. Thus, one of the strategies of backgammon: the priming game.