Timbre is the distinctive quality of the sound produced by different sources or instruments: it is what distinguishes a middle C played on a violin from one played on a piano. It's the least understood of musical properties, but is central to the way we respond to music, which after all is not just about pitches and harmonies annotated on a score but about the quality of the sound with which they are voiced. When symphony orchestras play lush interpretations of rock music, we learn (if we can bear it) that a badly amplified Fender Stratocaster can sometimes say much more than the precise, clean intonation of a 40-piece string section.

Some characteristics of timbre come from the mix of overtones in the sound, and some comes from the rise and fall of loudness as it is played. But timbre is not in fact a single musical attribute at all – it's a dumping ground for all the aspects of sound we can't conveniently identify as pitch, rhythm or tempo. Even officially it is defined more by what it isn't than by what it is. If we understood it better, both in acoustic and in psychological terms, perhaps we might make more systematic use of it in music.

Some composers have tried to do that, blending the sound of conventional instruments to make new, composite timbres. Schoenberg and Webern experimented with what they called 'sound colour melody', in which a single melody was parcelled out between several instruments so that each played just a few notes in succession. The result can be fascinating to listen to, but no one could work out how to deploy timbre in the same way as we do pitch and rhythm, or even if that is possible.