Nummer 2 for thirteen instruments (also called Opus 2 for thirteen instruments) is a composition written in 1951 by the Belgian composer Karel Goeyvaerts.
Nummer 2 has been claimed as the first "total serial" composition.[1] though the same claim has been made for Milton Babbitt's Three Compositions for Piano (1947), which predates Goeyvaerts's work by four years.[2]