In the early days of quantum mechanics when the idea that observables are represented by noncommuting operators was still new and strange, some people spoke of quantum observables as "quantities whose values are q-numbers" — the notion of "q-number" being meant to suggest noncommutativity — as opposed to "quantities whose values are c-numbers," i.e., ordinary complex-valued quantities whose algebra is commutative. One still finds the terms q-number (rarely) and c-number (more frequently) in the physics literature; in particular, to say that an operator is a c-number is to say that it is a scalar multiple of the identity. (E.g., "The commutator of A and B is just a c-number.")