In mathematics, especially in set theory, the terms, subset, superset and proper (or strict) subset or superset are used to describe the relation, called inclusion, of one set being contained inside another set. Informally, every element belonging to subset A also belongs to superset B, but there may be elements belonging to B that do not belong to A (see diagram to right).
A box containing the parts known as a Network and usually a Ringer. The Network consists of proper apparatus to boost speech sounds for transmission to the telephone line, to separate undulating currents to the Receiver, and to signal incoming calls if so equipped.
The designation the National Exchange Carrier Association uses to categorize its members according to size. Subset 1 includes Bell operating companies, subset 2 includes the large independent local exchange carriers with $40 million or more in annual telephone revenues and subset 3 includes all other local exchange with less than $40 million in annual telephone revenues.