O magnum mysterium was written in the autumn of 1986 for the choir of Westminster Under School, London, as a gesture of thanks for taking such good care of my son Orlando. It is a simple serial piece, scored for boys' or womens' voices in three parts, accompanied by the organ. "Complemented" is a more exact word than "accompanied", because the organ plays only one phrase - a compressed version of the piece's eleven-note series in the form of six two-note chords (the last of which repeats the initial note C). The series remains untransposed; harmonic variety is created by putting it in different pairings. After the brief appearance of the organ there is a brief climax on the word  admirabile; then the solo treble (or soprano) reappears, introducing a partial recapitulation of the opening. A celebratory Alleluia presents the entire series as a unison melody sung by the whole choir, and the piece ends with the solo treble's intoned C which, for the final note, drops to A - the only note not yet used in the piece.