In the Middle Ages, church music mostly used a 3:1 metre. Music with a 2:1 metre — indicated by a ‘C’ symbol (actually a half circle) that we today confusingly refer to as common time, but was then known as tempus imperfectum cum prolatione imperfecta — was rather less common. As the Latin title indicates, music in two or four was considered imperfect, and music in three perfect (owing possibly to its association with the Trinity). More often than not, this duple metre would itself be divided into two groups of three, like playing in 6/8 today. This was indicated by a dot after the ‘C’.

For some reason we increasingly do not divide by three. Music remains something of an exception, even though 4/4 has become the default time signature. Two, three and four seem to be the most natural divisions. Any higher than that is significantly less useful. We see this in compound metres — 12/8, for example, is divided into four groups of three quavers (3+3+3+3/8). And when presented with an irregular time signature like 7/4 or 13/8, what we usually do is turn it into an additive metre, where it is subdivided into twos, threes, and fours. So 7/4 might become 3+2+2/4 and 13/8 might become 3+3+3+4/8.

We clearly like to think in twos and threes, and so to have any system that uses only in one or the other would be a limited system. I can understand frustrations with the imperial system, but I am more frustrated by the inability to divide by three in the metric system. The frequent need for the number ten is particularly annoying as it does not subdivide by three or four, but requires either a subdivision by two or five (5+5 or 2+2+2+2+2) or a combination of twos and threes, rather like counting in 10/8. (But maybe this is just me — admittedly, very few others seem to be frustrated by this!)

I’ll end with some John Dunstable. In three, of course: