 Clef combinations played a role in the modal system toward the end of the 16th century, and it has been suggested certain clef combinations in the polyphonic music of 16th-century vocal polyphony are reserved for authentic (odd-numbered) modes, and others for plagal (even-numbered) modes, but the precise implications have been the subject of much scholarly debate.Music can be transposed at sight if a different clef is mentally substituted for the written one.For example, to play an A-clarinet part, a B-clarinet player may mentally substitute tenor clef for the written treble clef, and concert-pitch music in bass clef can be read on an E instrument as if it were in treble clef, although the keys and octaves of the notes will still need to be adjusted.