It's called permutations and combinations in the math world. Permutations are where order matters, combinations are where it doesn't.
http://betterexplained.com/articles/...-combinations/
In your HST example, order doesn't matter, so it's a combination. And specifically how many combinations of r objects (2 in this case) from a set of n objects (12 in this case). Without getting into too much detail or terminology, the answer is 66.
12 · 11 ÷ 2 · 1 = 132 ÷ 2 = 66
Another example: if you wanted to know the number of different four patches you could make with 20 fabrics, it would be:
20 · 19 · 18 · 17 ÷ 4 · 3 · 2 · 1 = 116,280 ÷ 24 = 4,845
Have fun!
