Plainchant is a type of liturgical music where religious texts are sung to a single unaccompanied line.

Whatever your verdict on Christianity, you can’t argue that it hasn’t inspired a dazzling diversity of devotional expression in music: from Renaissance motets, through Bach cantatas, Anglican hymns, Black American spirituals and Gospel music, to those limply syncopated, very soft rock hymns beloved of today’s charismatics.

But plainchant – or plainsong – is the oldest, probably dating from before the separation of Christendom into Western and Eastern branches, which may go some way towards explaining why it’s also one of the most broadly popular.

Just how old plainchant might be is difficult to say. The first concrete examples of written chant, notated on four-line staves with figures that look like conga-lines of beetles and tadpoles, date from the 11th century. The first serious theoretical writings about chant appeared a couple of hundred years earlier. Before that it gets messy.

With no even half-reliable system of notation in existence, it’s difficult to tell what kind of musical phenomenon the early Christian authorities were talking about. But it seems fairly clear that the preferred kind of chant was a single-line melody, rhythmically free (ie following the rhythms of speech), and definitely without any contributions from ‘heathen’ instruments.

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The fact that similar forms of chant exist in Eastern Orthodox and Jewish worship suggests common origins in something much older.

A sense of ancientness, of continuity throughout the ages, seems to be the key to plainchant’s enduring appeal. When, for his Mass in B minor, Bach based the ‘Credo’, the central assertion of faith, on the old Latin ‘credo’ plainchant, he was saying that what he believed, countless others had believed before him.

Listening to the old chant singing through his athletic, joyous counterpoint is also a reminder of how pliable plainchant is. In Britten’s Curlew River the chant ‘Te lucis ante terminum’ fuses with sounds derived from Japanese Nôh theatre to produce something fresh, yet reassuringly old.

The same could be said for the ecstatic string threnody that opens and closes James MacMillan’s The Confession of Isobel Gowdie: in the background, the old funeral chant ‘Lux aeterna’ stirs, just on the threshold of audibility. Something in us, it seems, still yearns for that sense of connection to – well, what?

This article was first published in the November 2013 issue of BBC Music Magazine