Cantus firmus is a fixed tune around which polyphonic choral music is developed.
Cantus firmus: ‘firm melody’ – if that sounds a reassuring concept, it was probably meant to be.
Polyphony – the complex interweaving of independent voices – took off in European music after the invention of musical notation. Improvised polyphony is possible, but it’s much easier to contrive when one musician (the composer) works all the strings; and for that, you really need to write things down.
In turn, weaving complex strands is simpler when you have something relatively firm and steady to weave them around. And that’s where the cantus firmus comes in.
You take a pre-existing melody: a plainchant, a made-up chant-like line, or perhaps even secular song, and you have it sung in long-held, slow-moving notes, usually in the lower voice.
In that respect it’s virtually the opposite of the familiar tune-plus-accompaniment texture of so much Western music, from opera to pop.
Cantus firmus reached the heights of abstract rarefication in the writings of theorists like Gioseffo Zarlino (1517-1590). But at the same time reaction was brewing – the new emerging medium of opera had no place for firm, orderly stability.
Yet that towering figure of early Italian opera, Monteverdi, also created a splendid, easily comprehensible introduction to the idea of cantus firmus in his famous Vespers of 1610.
The movement entitled ‘Sonata sopra Sancta Maria’ is a wonderfully florid concerto for early baroque orchestra, but the still centre around which the dancing, singing counterpoint turns is the first phrase of the old plainchant, ‘Sancta Maria, ora pro nobis’, sung out at spacious intervals by a group of voices labelled ‘Cantus’.
This gave the cantus firmus new life. Slow-moving, plainchant-based lines sing out calmly and securely through the joyous, athletic vocal and instrumental polyphony of the Credo from Bach’s Mass in B minor – apt for music dealing with the affirmation of faith.
And Bach’s own instrumental Chorale Preludes, which weave so many varieties of counterpoint around Lutheran hymn-tunes, show the notion of the musical ‘firm’ centre entering a new age, a new style, and a new religious confession.
And despite the manifold upheavals of the 20th century the idea has proved remarkably enduring: Stravinsky, Honegger, Hindemith, Maxwell Davies… The yearning for some kind of ‘firm melody’ endures.
This article was first published in the August 2012 issue of BBC Music Magazine