An accordion (French: accordéon, German: Handharmonica "hand harmonica" and Ziehharmonica> "tooth harmonica"), is a small portable free-reed wind instrument with a keyboard, the smallest representative of the organ family.
Sound is made by a thin metal ribbon, a reed, that is held at one end and free at the other, like a ruler on the edge of a table top. The reed is fitted inside a holder plate, air is drawn through the hole in the holder, the reed vibrates, producing sound.
The first modern accordion was a 10-button accordion, invented in 1829 by Damian, in Vienna, which had the 7 notes of a major scale, and consequently only played in one key [and its related keys]. These accordions are still played today and are called many things, Cajun accordions, melodeons, one-row, diatonic accordions, and so on. They are single-action instruments, where as a rule each button produces two different notes, one when pulling the bellows outwards, one when pushing it inwards. The notes are arranged much like on a harmonica.
The accordion consists of a bellows of many folds, to which is attached a keyboard with from 5 to 50 keys. The keys on being depressed, while the bellows are being worked, open valves admitting the wind to free reeds, consisting of narrow tongues of metal riveted some to the upper, some to the lower board of the bellows, having their free ends bent, some inwards, some outwards. Each key produces two notes, one from the inwardly bent reed when the bellows are compressed, the other from the outwardly bent reed by suction when the bellows are expanded. The pitch of the note is determined by the length and thickness of the reeds, reduction of the length tending to sharpen the note, while reduction of the thickness lowers it. The right hand plays the melody on the keyboard, while the left works the bellows and manipulates the two or three bass harmony keys, which sound the simple chords of the tonic and dominant.
Related instruments include the concertina and the melodeon.
The piano accordion was developed in Europe in the late 1800's. Familiar to everyone who has ever seen Lawrence Welk, the right hand is laid out like a piano keyboard, so a piano player could play it. The left hand plays in a forest of up to 120 buttons which play bass notes and various chords. The instrument was named and popularized in the United States by Count Guido Deiro who was the first piano accordionist to perform in Vaudeville. He is credited with making the first recordings of the instrument in 1908, also with making the first radio broadcast of the accordion in 1921 and the first sound motion picture featuring the accordion, Vitaphone 1928.
Another type is the chromatic accordion. Usually these have buttons instead of piano keys, but they have the same 12-note Western scale as a piano accordion. Also their keyboards are laid out so that playing a tune has the same fingering in any key [unlike the piano accordion].
Piano accordions and chromatic accordions are double-action instruments: each key or button plays the same note or chord, whether the bellows are being pulled out or pushed in.
Free bass accordions, favored by classical accordionists, have a left-hand button board with individual bass notes over several octaves, rather than the single octave of bass notes and the preset chords provided by the traditional "stradella" left-hand button system. There are "convertor" accordions offering both systems in one instrument.
Many folk cultures have their own flavor of the accordion, including the Russian bayan, Alpine helikon instruments, North Mexican conjunto accordion, Louisiana Cajun accordion, Irish 2 row b-c type instruments, etc. These can have either a unique note layout, a different sound, or all of the above.