Bisonoric button accordions are generally diatonic: they contain the notes of only one or a few keys, often supplemented by selected missing chromatic notes.
For this reason, “bisonoric” and “diatonic” are often used as synonyms, although they do not mean the same thing.
Producing different notes on the push and pull made it possible to obtain twice as many notes from the same mechanism, simplifying construction and reducing costs.
One-row accordion
The simplest diatonic accordions have one row on the treble side and only two bass buttons plus an air button. Despite their early origins, they remain important in Irish music as the one-row melodeon and in American Cajun music as the Cajun accordion.
With only two bass buttons or spoons, just two chords can be played, for example C major and G major. This creates a simple, direct musical style.
The treble layout resembles the Richter system used on harmonicas, allowing harmonica players to transfer many familiar patterns. Since each instrument is limited to one key, models are made in several keys.
Two-row accordion – Vienna model
The note arrangement of two-row accordions has changed very little over the decades. The so-called Vienna model is widespread.
These instruments often have only eight bass buttons, each producing different bass notes and matching chords on the push and pull.
German Club model
The term diatonic accordion often refers to the German Club model. Its treble side has two rows assigned to two keys, usually C/F, plus a third row containing missing chromatic notes.
An important identifying feature is the unisonoric button in the second row, which produces the same note in both bellows directions. Vienna models do not have this button.
The bass side also provides an additional chord. On Vienna models, one button produces the same chord in both directions.
Styrian accordion
The Styrian accordion remains popular in folk music, especially in Austria, South Tyrol, the Czech Republic, Slovenia and Bavaria.
Its main distinction is the use of helicon basses: very low bass notes reminiscent of the helicon tuba. Like Club models, Styrian accordions also have unisonoric buttons that produce the same note in both bellows directions.
Accordéon diatonique and melodeon
The accordéon diatonique is still widespread in France, for example in Brittany, and is also popular with younger players.
It is essentially a Vienna model, often with a third row of accidentals that adds notes not included in the principal keys. In France the common tuning is G/C.
The English and Irish melodeon is similar, but the two keys are often only a semitone apart, commonly B/C. This makes the full chromatic scale available. Chromatic harmonicas with a slide use the same principle.
