Minus 13 is easiest described as a multi-player shut the box. Players take turns rolling 3 dice and removing colored chips from hexagonal pyramid that is made from six triangular parts. The players total up the numbers on the dice and remove the colored chip that sits on the corresponding number. If there is no chip left on that number then the player takes any number of chips from numbers that add up to the total of the roll. When a 13 is rolled the player must take chip that sits on top of the pyramid that is worth negative 13 points. If a player is unable to take chips that add up to his or her number (ie. another 13 is rolled) then they must pass one of their colored chips to the person on their left. In this way the "minus 13" chip can be passed on.
At the end of the game, each player rolls dice to determine how many points each colored chip is worth. The player holding the "minus 13" chip subtracts the appropriate amount of points and the person with the most points wins.