 The realisation from the 'closed' (unrealised) form can be effected by placing the page in front of a mirror, thus upside down, and beginning with the already progressing first voice.The Canon a 2 'Quaerendo invenietis' from J. S. Bach's "The Musical Offering", BWV 1079, is a fine example of the process.In its original closed form the alto clef and an upside-down bass clef indicate both the mirror procedure and the appropriate pitches of the voices for the purpose of realisation.