Victor Charles Mahillon, curator of the instrument collection at the music conservatory in Brussels, divided the collection into four main groups for the 1888 catalog: strings, winds, drums and other percussion. His system followed an ancient system used in India in the first century B.C. Erich von Hornbostel and Curt Sachs published a system in 1914 that expanded on Mahillon’s classification scheme. Later, Sachs added a fifth category for electronic instruments. These classifications are the most widely used today.
Chordophones include instruments in which sound is produced by a vibrating string. The string can be plucked (as with a banjo or koto), struck (as with a piano), or bowed (as with a violin). A resonating chamber is typically used to amplify the sound.
Aerophones produce sound by a vibrating column of air. The air can be created directly by the mouth blowing across an opening or by squeezing a bellows. Winds, brass, accordions and harmoniums all fall within this category.
Membranophones produce sound through the vibration of a stretched membrane. Most of these instruments are drums, but the kazoo, in which a membrane vibrates in sympathy with the singing of the human voice, also falls under this category.
With Idiophones, the material of the instrument itself vibrates to create sound when struck, scraped or shaken. Xylophones, maracas, chimes, gongs, and guiros are among the endless array of percussion instruments that populate this category.
Electrophones are of more recent vintage and include instruments that produce sound by electronic means. Theremins, turntables and synthesizers are the obvious examples, but as you move into more sophisticated electronic instruments that use samples to emulate traditional instruments like the piano or the clarinet, the classification becomes fuzzy. If it sounds like a piano, is it a piano or a synthesizer?
As with any classification system, there are objects that do not fit neatly into one category. Some scholars feel that tuned idiophone instruments such as xylophones, Indonesian gamelan instruments, and African mbiras (sometimes called thumb pianos, in which the player flexes strips of metal attached to a sound board) should comprise a separate category of “lamellophones” because the composite sound of the instrument is not totally self-contained like other idiophones.
Then there are idiotphones: cell phones in the hands of certain people. But that’s another story…