Term normally applied to atonal mus., meaning a small group of notes (a cell or a unit) which the composer or the analyst of the work concerned deems to be of structural significance, e.g. the notes B–C–F in the 4th of Webern's 5 Movements for str. qt. In a serial work the series or part of it may be considered as the set. And so can the opening notes of Beethoven's 5th Sym.!