The Austrian musicologist, Erich Moritz von Hornbostel (1877-1935), and his German colleague, Curt Sachs (1881-1959), proposed in 1914 a system of classification for musical instruments which has been criticized and changed in details through the years, but never supplanted. The following chart — of my own design — (which depicts the position of the free-reed instruments in relation to the entire body of musical instruments) is based upon their work.

Chordophones Stringed Instruments			Bowed		Violin
					Viola
					Cello
					Contrabass
			Plucked		Guitar
					Banjo
					Ukulele
					Harp
					Harpsichord
			Struck		Hammered Dulcimer
					Piano
Aerophones Wind Instruments Aerophones Wind Instruments	Pipe Aerophones pitch determined by pipe length	Edge Instruments	Whistle Flutes		Whistle
					Recorder
					Organ Flue Pipes diapasons, flutes, mixtures, etc.
			True Flutes		Jug
					Panpipes
					Flute
					Piccolo
		Reed Pipe Instruments	Single Reeds		Clarinet
					Saxophone
					Single Reed Bagpipe
			Double Reeds		Oboe
					Bassoon
					Double Reed Bagpipe
		Brass Instruments	Without Valves		Conch shell
					Animal Horn Shofar
					Didjeridu
					Bugle
					Trombone
			With Valves
					Trumpet
					Cornet
					French Horn
					Euphonium
					Tuba
	Free Aerophones pitch not determined by pipe length	Beating Reed Instruments reeds strike against another object	Single Reed		Organ Reed Pipes hautbois, fagotto, chalumeau, krummhorn, clairon, trompette, trompette en chamade, trombone, tuba, etc.
			Double Reed		Human Voice (*1)
		Free-Reed Instruments reeds vibrate freely without striking anything	Unframed Reed	Wind Blown	Bull-Roarer
					Aeolian Harp
				Mouth Blown	Leaf Instrument
				Mouth Blown & Plucked	Jew's Harp
			Framed Reed	Mouth Blown	Shêng
					shô
					Khaen
					Harmonica
				Hand Blown
					Concertina
					Bandonéon
					Bayan
					Accordion
					Indian Harmonium
				Foot Blown
					Harmonium
					Reed Organ
					Pedal Concertina
				Mechanically Blown
					Barrel Organ
					Orchestrion
					Pedal Reed Organ
					Electric Chord Organ
Percussion Instruments	Idiophones body of instrument vibrates	Pitched	Struck		Triangle
					Bell
					Gong
					Cymbal
					Xylophone
					Marimba
					Celesta
			Rubbed		Prayer Bowls
					Glass Harmonica
			Plucked		Music Box
					Kalimba, Mbira
		Unpitched	Struck		Slit Drum
					Castanets
			Shaken		Rattle
					Jingles
	Membranophones membrane vibrates	Determinate Pitch	Struck		Timpani
					Roto Toms
		Indeterminate Pitch	Struck		Snare Drum
					Bass Drum
					Bongos
					Congas
					Tambourine
			Rubbed		Friction Drum
			Blown		Kazoo
Electrophones		Electric/Acoustic Instruments			Electric Guitar
					Electric Bass
					Fender Rhodes Electric Piano
					Electric Violin
		Electronic	Electromagnetic Instruments		Theremin
					Ondes Martenot
					Electric Organ
					Synthesizer
			Digital Instruments		MIDI Keyboard
					MIDI Wind Controller
					MIDI Drum Machine
					MIDI Guitar
					MIDI Accordion

In essence, the aerophones (wind instruments) use air as the primary vibrating medium for the production of sound. The aerophones are divided into two subsets: the pipe aerophones and the free aerophones. The pipe aerophones (the oboe, bassoon, clarinet, saxophone, etc.) regulate pitch by changing the length of the pipe; the size of the reed does not change. On the other hand, the free aerophones regulate pitch by changing the length (and thickness) of the reed. Many free aerophones do not use pipes (accordion, concertina, etc.), but for those instruments that do have pipes (such as the shêng, shô, khaen and organ reed pipes), pipe length has no effect on the pitch, although the pipe does function as an acoustical reinforcement for the sound.

The free aerophones can be further divided into two subsets: beating reeds and free-reeds. Organ reeds are referred to as beating reeds because the tongue is larger than the shallot opening and therefore beats against it. In a free-reed, on the other hand, the tongue is smaller than the opening and so vibrates through rather than against it. Most tongues of free-reeds are made from metal, but tongues of primitive free-reed instruments, like the naw, are made from cane.

The free-reed instruments are divided into two more subsets: the unframed reed and the framed reed. The simplest reed instruments are those which have no openings to channel the wind or frames within which the reeds can vibrate. The aeolian harp, a musical instrument played by the wind, can be convincingly classified as a free-reed instrument, although it is customarily categorized as a chordophone. The instrument, named after Aeolus, the Greek god of wind, is made of a wooden sound box loosely strung with ten or twelve gut strings varying in thickness and elasticity, usually tuned in unison. In the wind they vibrate in aliquot parts (i.e., in halves, thirds, fourths, etc.) thus sounding the octave, 12th, second octave, and succeedingly higher harmonics of the string's fundamental note, which is silent.

According to legend, King David hung his kinnor (a kind of lyre) above his head at night to catch the wind. In the tenth century, Dustan of Canterbury was charged with sorcery when the wind produced sound from his harp. The first known Aeolian harp was constructed by the Jesuit priest and scholar, Athanasius Kircher (1601-1680), and was described in his Musurgia Universalis (1650). The instrument became popular in Germany and England during the romantic period. Two attempts to devise a keybard version using a bellows were the anémocorde (1788) by Johann Jacob Schnell and the piano éolien (1837) by M. Isouard. One familiar form of the aeolian harp is the musical tones produced by the wind in telephone wires, which can be amplified by placing the ear to the side of the pole. (*2)

The Jew's harp (sometimes called Jaw harp) is another instrument which cannot be conclusively categorized. The first mention of this instrument in this context was made by Sebastian Virdung, who grouped the Jew's harp together with rustic instruments, such as hunting horns, bird calls, and bells in his Musica Getutscht (Basel, 1511). About a century later, in Theatrum Instrumentorum (1618), Michael Praetorius classed the Jew's harp with the hurdy-gurdy, the viele, the horn, the triangle, and the bell. A far more precise approach to classification was made by the famous seventeenth-century musicologist Marin Mersenne (although he appeared to have two views on this matter): In Traité des instruments de Musique (1640), Pierre Trichet stated that Mersenne regarded the Jew's harp as a "pneumatic" instrument, since breath participated in producing its sounds, but in Harmonie Universelle (1646), he termed it a chromatic or percussion instrument, because breath alone, without striking, does not make it yield any sound.

Modern attempts to classify the Jew's harp have not settled the issue. Curt Sachs invented the term "plucked idiophone" for the instrument, which produces sounds due to the rigidity and elasticity of the material from which it is made. More recently, Frederick Crane and Ole Kai Ledang have returned to Mersenne's original view and have classified the Jew's harp as an aerophone, arguing that full functioning of the instrument occurs only when a stream of air moves past its tongue. (*3)

Another example of the unframed reed is the primitive bull-roarer. A spatulate stone, bone, or board, sometimes carved in the shape of a fish, is tied through a small hole to a string, which in turn is attached to a stick; when the instrument is whirled around, it produces a sound by its disturbance of the air. Primitive societies believe this instrument has magical properties. This instrument appeared in the 1986 movie Clan of the Cave Bear.

Another primitive free-reed instrument is the leaf (called bilu), which can be heard in some traditional Chinese music ensembles. A leaf, or a long blade of grass, is stretched between the sides of the thumbs and tensioned slightly by bending the thumbs, thereby raising or lowering the pitch. The tone of the instrument can be modified by cupping the hands so as to provide a resonant chamber. Like the aeolian harp, the reed is secured at each end and the center length is allowed to vibrate about its fixed ends. The quality of sound can also be altered by changing the thickness of the leaf. This instrument is extremely hard to classify exactly, since, depending on the shape of the cavity created between the thumbs, the grass reed can be proved to be both beating and free, or neither.

All other free-reed instruments have a frame through which the tongue vibrates. The framed free-reed instruments can further be divided into four subsets: mouth blown, hand blown, foot blown and mechanically blown.

This classification is convenient, but not necessarily logical, as some instruments may belong to more than one class. As mentioned above, the Jew's harp may be classified as an aerophone or a plucked idiophone. The aeolian harp may be classified as an aerophone or a chordophone. In addition, the organ belongs to both the edge instruments and reed pipe instruments. The violin can be categorized as a bowed chordophone or a plucked chordophone, depending on whether it is player arco or pizzicato. The tambourine is a membranophone in so far as it has a skin head which is struck; but, if it is only shaken so that its jingles sound, it should be classed as an idiophone, for in this case the skin head is irrelevant.

The accordion may also belong to more than one family. The American accordionist/composer Guy Klucevsek has written a piece for solo accordion, Eleven Large Lobsters Loose In The Lobby (1991) which does not use the reeds of the accordion. The performer produces sounds by clicking the register switches, tapping the keys, and other percussive means. In this piece the accordion is used as an idiophone and not as a free-reed.

Twentieth-century hybrids like the Cordovox are properly categorized as an instrument which combines more than one category: it is simultaneously a free-reed instrument and also an electromagnetic instrument. Of course the reedless accordion -- the MIDI accordion -- cannot be classified as a free-reed instrument since it has no reeds. It is solely a digital instrument.

The free-reed instruments can be further subdivided according to various features such as:

• Direction of wind pressure (positive, negative or both)
  
• Playing method (lips, fingers over holes, keys, buttons, and mechanical means such as cogs in barrels, punched cards, etc.)
  
• Scale (pentatonic, diatonic, chromatic)

The Australian musicologist and harmonium enthusiast, Diarmuid Pigott -- curator of the Harmonium Home Page -- created the following chart categorizing the free-reed instruments in greater detail, on which I have expanded upon.

The Free-Reed Family of Aerophones By Diarmuid Pigott

Air supply	Pressure	Played	Group	Scale	Notes
Mouth Blown Mouth Blown
	+/-	Fingers over holes	Asian mouth organ	Various: Pentatonic Diatonic Chromatic	Chinese Shêng Japanese shô Laotian Khaen
		Lips and Tongue	Harmonica or Mouth-Organ	Diatonic	Marine Band ("Blues Harp")
					Tremolo Harmonica, Echo harmonica
				Chromatic	Chromonica, Chromatitica
				Hybrid	Koch Chromatic Marine Band
				Chordal	Chord Harmonica
	+	Lips	Pitch Pipe	Various	Used as tuning note for choirs, etc
		Flute-like	Klui	?	From Thailand: the only free-reed resonated flute
		Trumpet-keyed	Shalmei	Diatonic	Tyrolian Many Belled Trumpets - solo instrument
				Chords	Tyrolian Many Belled Trumpets - accompaniment
		Buttons	Symphonium	Chromatic	Predecessor of Wheatstone's Concertina
		Lever Action	Toy Instrument	Diatonic	Toy "Clarinet"
		Keyboard	Melodicas	Chromatic	Hohner Melodica
Hand Blown Hand Blown	+/- +/-	Buttons	Concertina	Diatonic	Single-Action German or "Anglo" Concertina
				Various	Single-Action Multi-Row Anglo Concertina
				Chromatic	Double-Action English Concertina
					Double-Action McAnn System Concertina
					Double-Action Triumph/Crane System Concertina
			Bandonéon and Chemnitzer Concertina	Diatonic	Single-Action Bandonéon
					Chemnitzer Concertina
				Chromatic	Double-Action Bandonéon
			Melodeon	Diatonic	Melodeon
			Diatonic Button Accordion	Diatonic	Organetto Abruzzese
				Bi-Diatonic	Cajun
					Irish
					Italian
					Polka
			Chromatic Button Accordion	Chromatic	Russian Bayan
		R. H. Keyboard L. H. Buttonboard	Accordion	Chromatic	Early Single-Action English and French
					Standard Stradella-Bass Piano Accordion (*4)
					Modern Free-Bass Piano Accordion
					Reuther's Uniform Keyboard
		Keyboard/Frets	Melophone	Chromatic	Accordion in the form of a cello/guitar
	+	Buttons	Lap organ	Chromatic	James A. Bazin, U.S.A. 1836 (Early Models)
		Keyboard	Harmonium	Chromatic	Indian Droned Harmonium
			Lap Organ	Chromatic	Bazin, U.S.A. (Later Models)
Foot Blown
	+/-	Keyboard	Peaseley	Chromatic	Aaron Merrill Peaseley U.S.A. 1818 (*5)
	+
		Keyboard	Orgue Expressif	Chromatic	Grenie, Paris 1803
			Physharmonica	Chromatic	Haeckel, Vienna 1818
			Seraphine	Chromatic	Green, London 1831
			Harmonium	Chromatic	Debain, Paris 1840
			Vocalion	Chromatic	Farmer, Harrow 1872
		Foot Buttons	Pedal Concertina	Diatonic	Belgian Instrument
	-	Keyboard	Experimental	Chromatic	Alexandre, Paris 1835
			American Organ	Chromatic
					Melodion, US 1835
Mechanically Blown	+	Cogs in Barrel / Punched Cards	Barrel Organ	Diatonic/ Chromatic	Barrel Organ
	+	Cogs in Barrel	Orchestrion	Chromatic	Mälzel's Panharmonicon, 1804
	+/-	Keyboard & Pedalboard	Pedal Reed Organ	Chromatic	Pedal Reed Organ
	+/-	R.H. Keyboard L.H. Buttonboard	Chord Organ	Chromatic	Electric Chord Organ

As noted above, the free-reed instruments can be classified according to their type of musical scale into two categories: diatonic and chromatic instruments, (*6) although some Oriental instruments can play only a pentatonic scale. The diatonic instruments are limited to the tones of a standard major or minor scale without the chromatic intervals; therefore, diatonic instruments can play in only one key. (Bi-diatonic instruments can play in two keys.) On the other hand, chromatic instruments can play all twelve tones of the chromatic scale.

Free-reed instruments can still be further divided into single-action instruments and double-action instruments. The single-action instruments have two different pitches per button; that is, one pitch sounds when the bellows are opened and another pitch sounds when the bellows are closed. The double-action instruments sound the same note on the press as on the draw. All piano-accordions are chromatic double-action instruments.