An accordion 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 free-reed instrument was the Chinese sheng (笙), which is mouth-blown. It is thought that a traveler to China in the 1800s brought this idea back to Europe.

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 was patented on January 14, 1854 by Anthony Faas.

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 and has become the most common type of accordion nowadays. 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, though the keys are smaller than on a piano. 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.

The left hand layout usually features six rows, the second row buttons consist of the Bass and is ordered in quints, the first row buttons are one third up relative to the second row. The major chords are in the third row, the fourth row consists of the minor accords, the fifth row houses the seventh chord and finally the sixth row has the diminished seventh chords.

The layout can be roughly described by this ASCII Art:

 ... C    G    D    A   E   B   F#  C#  G#  D#  A#   F    C    ...
 ... Ab   Eb   Bb   F   C   G   D   A   E   B   F#   C#   G#   ...
  ... ab   eb   bb   f   c   g   d   a   e   b   f#   c#   g#   ...
   ... abm  ebm  Bbm  fm  cm  gm  dm  am  em  bm  f#m  c#m  g#m  ...
    ... ab7  eb7  Bb7  f7  c7  g7  d7  a7  e7  b7  f#7  c#7  g#7  ...
     ... abd7 ebd7 Bbd7 fd7 cd7 gd7 dd7 ad7 ed7 bd7 f#d7 c#d7 g#d7 ...

Depending on the price, size or origin of the instrument, some rows may miss completely or the layout is slightly changed. In most russian layouts the diminished seventh chord row is moved by one button, so that the diminished seventh C chord is where the diminished seventh F chord is in this ascii graphic, in order to achieve a better reachability with the forefinger.

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. The buttons are ordered chromatically in three rows, one row up/down means one halftone up/down, one button up/down in the same row means 3 halftones up/down. Larger chromatic accordions can have up to three auxiliary rows, with secondary buttons playing the same tones that already appeared in the first three rows. This layout makes transforming songs into other keys much easier than on the piano accordion. The chromatic accordion is definitely the choice for classical music, as a lot of more buttons than piano keys can be packed on the same space. Therefore artists can play intervals of up to two octaves using only one hand, this is especially important for pieces that include more than two voices. There are two different layout systems, the C layout and the B flat layout. If you turned a C layout keyboard on its head you would have a B flat layout and vice versa. The B flat layout is preferred for classical music, and is very common in Eastern Europe whereas the C layout is common in the western part, particularly in France.

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, Bariton bass or Melody 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 and works exactly the same way the right hand on the chromatic accordion does. There are "converter" 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.