A unit is an element formula_28 having a multiplicative inverse; in this case the inverse is unique, and is denoted by formula_38. The set of units of a ring is a group under ring multiplication; this group is denoted by formula_39 or formula_40 or formula_41. For example, if "R" is the ring of all square matrices of size "n" over a field, then formula_39 consists of the set of all invertible matrices of size "n", and is called the general linear group.