The number of possible ways to choose combinations from total items is denoted using two parentheses with the variables and . The subscript is included to indicate that the expression represents combinations where *order does not matter* as opposed to permutations where order does matter. Typically, the notation is used in an expression like this:

In plain language, this represents the total number of combinations from choosing items from a collection of items. See permutations for where order does matter.

The combination formula describes the possible combinations of r elements out of a group of n elements where order does not matter.