In each game, the first team can be any one of the 12 ,
and the second team can be any one of the remaining 11 .
Total number of ways to match them up = (12 x 11) = 132 .
BUT ... Every game can be described as two different match-ups ...
either as ' A ' playing ' B' , or as ' B ' playing ' A '.
Although there are 132 ways to make a match-up, every possible
pair of teams shows up twice in that list.
So in order for every team to play every other team, there are
only (132 / 2) = 66 different games required.
