Is a pair of twins two or four persons?

Pair is derived through French from Latin paria (“equal”), and signifies “two.” Twin comes from a root meaning “two” and means the same thing literally. As a singular noun twin is generally defined as “one of two children or young brought forth at one birth.” The plural form twins is defined as “two children or young brought forth at one birth.” Logically speaking, then, a pair of twins should mean four persons. But it does not and never did. Nobody uses the expression to mean more than two. When a speaker or writer employs the phrase pair of twins he invariably means exactly what he would mean if he said merely twins. “Mrs. Jones has a pair of twins” and “Mrs. Jones has twins” are identical in meaning according to accepted usage. Since pair adds nothing to the sense, why not say simply twins? It cannot be questioned, however, that pair of twins is widely used in popular speech and has some literary sanction. In such, phrases pair is now followed by of, but formerly the of was omitted and people spoke of a pair gloves and a pair shoes. Pair of is followed by a noun plural in form even when not in meaning and this rule accounts for pair of twins.