To obtain the possible number of combinations of a number of elements:     ex. the possible combinations of  A B C D is 4! = 1x2x3x4 = 24             (! is called factoral)

-Multiply the numbers from 1 up to the total number of elements being considered. This yields the number of combinations possible with that number.

Another example is the number of possible combinations of X  Z :   2!=1×2=2      X Z, Z X

Another example:   X Y Z   3!=6      X Y Z, X Z Y, Y Z X, Y X Z, Z X Y, Z Y X

Try figuring the possible batting order combinations for a baseball team!!!