At various times, and in various different cultures, various schemes of musical instrument classification have been used.
The most commonly used system in use in the west today divides instruments into string instruments, wind instruments and percussion instruments. However other ones have been devised, and some cultures also use different schemes.
The oldest known scheme of classifying instruments is Chinese and dates from the 4th century BC. It groups instruments according to what they are made out of. All instruments made out of stone are in one group, all those made out of wood in another, those made out of silk are in a third, and so on.
More usually, instruments are classified according to how the sound is initially produced (regardless of post-processing, i.e. an electric guitar is still a string-instrument regardless of what analog or digital/computational post-processing effects pedals may be used with it).
Strings, percussion, and wind
The system used in the west today, dividing instruments into wind, strings, and percussion, is of Greek origin. The scheme was later expanded by Martin Agricola, who distinguished plucked string instruments, such as guitars, from bowed string instruments, such as violins. Classical musicians today do not always maintain this division (although plucked strings are grouped separately from bowed strings in sheet music), but there is a distinction made between wind instruments with a reed (woodwind instruments) and wind instruments where the air is set in motion directly by the lips (brass instruments).
There are, however, problems with this system. Some rarely seen and non-western instruments do not fit very neatly into it. The serpent, for example, an old instrument rarely seen nowadays, ought to be classified as a brass instrument, as a column of air is set in motion by the lips. However, it looks more like a woodwind instrument, and is closer to one in many ways, having finger-holes to control pitch, rather than valves. There are also problems with classifying certain keyboard instruments. For example, the piano has strings, but they are struck by hammers, so it is not clear whether it should be classified as a string instrument, or a percussion instrument. For this reason, keyboard instruments are often regarded as inhabiting a category of their own, including all instruments played by a keyboard, whether they have struck strings (like the piano), plucked strings (like the harpsichord) or no strings at all (like the celesta). It might be said that with these extra categories, the classical system of instrument classification focuses less on the fundamental way in which instruments produce sound, and more on the technique required to play them.
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