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 produced. 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, a old instrument 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.
An ancient system of Hindu origin, dating from at least the 1st century BC, divides instruments into four main classification groups: instruments where the sound is produced by vibrating strings; instruments where the sound is produced by vibrating columns of air; percussion instruments made of wood or metal; and percussion instruments with skin heads, or drums. Victor Mahillon[?] later adopted a system very similar to this. He was the curator of the musical instrument collection of the conservatoire in Brussels, and for the 1888 catalogue of the collection divided instruments into four groups: strings, winds, drums, and other percussion. This scheme was later taken up by Erich von Hornbostel and Curt Sachs who published an extensive new scheme for classication in Zeitschrift für Ethnologie in 1914. Their scheme is widely used today, and is most often known as the Sachs-Hornbostel system (or the Hornbostel-Sachs system).
The original Sachs-Hornbostel system classified instruments into four main groups: idiophones, such as the xylophone, which produce sound by vibrating themselves; membranophones, such as drums or kazoos, which produce sound by a vibrating membrane; chordophones, such as the piano or cello, which produce sound by vibrating strings; and aerophones, such as the pipe organ or oboe, which produce sound by vibrating columns of air. A later revision added a fifth category, electrophones, such as theremins, which produce sound by electronic means. Within each category are many subgroups. The system has been criticised and revised over the years, but remains widely used by ethnomusicologists and organologists.