The 2-row diatonic button accordion is an expansion of the earliest single-row accordions. The second row is usually similar in layout to the first, but transposed into a different key.
A 2-row "Vienna accordion" is arranged so that the keys are a fourth apart (like D/G, G/C, C/F etc.).
In Irish 2-row button accordions the rows are a semitone apart (like B/C or C#/D). This "British Chromatic" arrangement makes any note (but not any chord!) possible.
|2-row diatonic button accordion|
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