TITLE: Notation for: all subsets of size 2
QUESTION [13 upvotes]: How would one denote the set of all subsets of $A$ which have size $2$?
Would 
$$\binom{A}{2}$$
be a good idea?

REPLY [17 votes]: $\dbinom A 2$ is standard notation for the set of all size-$2$ subsets of a set $A$, in the usage of combinatorialists.

REPLY [7 votes]: In set theory it can be often denoted as $[A]^2=\{\{a,b\}:a,b\in A, a\neq b\}$.