Faced with the sheer variety of the world's instruments, from a violin to a gong to a bamboo pipe, the obvious way to sort them is by looks or by material: the wooden ones here, the metal ones there, the ones you blow apart from the ones you strike. In 1914 two scholars, Erich Moritz von Hornbostel and Curt Sachs, proposed a far better principle. They classified instruments not by how they appear or what they are made from, but by how they actually make their sound.

That single shift did the sorting properly. Under their scheme, everything that sings by way of a vibrating string belongs together, whether it is a harp or a piano or a lute, because the physics of the sound is the same even when the shape is not. Vibrating air columns form another family, taut stretched skins another, and rigid bodies that ring when struck yet another. The system was not built from nothing; it grew out of an earlier catalogue that Victor-Charles Mahillon had assembled for the collection at the Brussels Conservatory. But Hornbostel and Sachs turned a museum inventory into a universal grammar of sound production.

The scheme proved durable enough that musicologists and museums still lean on it more than a century later, and elastic enough to grow. When instruments arrived that made their sound electrically, a whole class the original authors had no reason to imagine, the framework simply added a fifth family for them, an expansion commonly credited to Sachs around 1940. That flexibility is the sign of a good idea: a way of organising the world that can absorb the things its makers never saw coming. Ask not what an instrument is made of but how it turns energy into a note, and every instrument that has ever existed, or ever will, already has a place waiting for it.