Last week I was in Rome, where I walked into a church one day to hear a group of about twenty nuns chanting an evening service. (I say ‘chanting’ because it wasn’t exactly singing, nor was it exactly speaking, but some melodious hybrid of the two.) There was a small group at the front who sang certain phrases, and then the rest would join in with responses in a pattern repeated many times as we sat there listening. ‘Santa Maria, madre di Dio, prega per noi …. ‘

It was quite fascinating from a musical point of view. First of all, I had never heard the ‘response’ group come in a major third above the others. I’ve heard various intervals used in sacred music, such as fifths and octaves, but never a third. For some reason it sounded curiously modern.

Even more interesting was the way the pitch of the responses ‘drifted’ downwards by a semitone during each repeated phrase. With every entry, the nuns began unerringly on the same note (‘middle C’), but by the end of the phrase they were a semitone lower, and by the end of longer phrases they were sometimes two semitones lower.

At first it seemed as if the ‘drift’ was a natural consequence of the voices tiring. But then I started to notice that the ‘drift’ began at exactly the same point in each phrase. It was repeated enough times that I could actually count the syllables before the drift began. I realised that what seemed accidental was actually very precise (though probably not planned).

It was usually about ten syllables in to the phrase that the drift began, and it never drifted further than one semitone during a short phrase, or two semitones during a longer one. Even the drift from one-semitone-down to two-semitones-down happened at a consistent point in the longer phrase. Then, with the next entry, everyone snapped back to the same razorsharp pitch they started with. Nobody looked at anyone else and there was no-one directing the ‘choir’. It was a perfect example of entrainment, the phenomenon whereby performers synchronise with one another unconsciously. What seemed like drifting was in fact beautifully co-ordinated, like a flock of birds making a slight curve in the air.