In the 17th Century the English mathematician John Wallis gave some meaning to negative numbers by inventing the number line. These days the number line and its cousins, notations that map numbers onto space such as tape measures, rulers and graphs, the petrol gauge in your car – are everywhere.

We don’t wonder: Is it natural? Is it cultural? Or worse – is it accurate? Musician Colin Reid reckons there’s more going on than meets the eye.