What is "a due"

A due in Italian or à deux in French is a musical direction meaning "for two". Most often seen in its abbreviated form a2, the marking signifies that on a staff that normally carries parts for two players, both players are to play the single part in unison. It is generally seen in scores and parts where two players or sections of the same instrument share a staff. The instruction a2 indicates that both players or sections should play the notes indicated, while primo and secondo (often abbreviated to 1. and 2. or I and II) indicate that only a single player or section should play while the other remains tacet.