If D shaking C's hand was actually different than C shaking D's hand, then we would count 12. But we just wanted to say, well, how many ways, they just have to meet each other once. And so we're double counting. So, A, B is the same thing as B, A. A, C is the same thing as C, A. A, D is the same thing as D, A. B, C is the same thing as C, B.