Accordion

An accordion is a musical instrument of the handheld bellows-driven free reed aerophone family, sometimes referred to as squeezeboxes.

Stradella bass system

The Stradella Bass System uses rows of buttons arranged in a circle of fifths; this places the principal major chords of a key in three adjacent rows. Each row contains, in order: A major third (the "counter-bass" note), the root note, the major chord, the minor chord, the seventh chord, and the diminished seventh chord.

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Depending on the price, size or origin of the instrument, some rows may be missing completely or in different positions. In most Russian layouts the diminished seventh chord row is moved by one button, so that the C diminished seventh chord is where the F diminished seventh chord would be in a standard Stradella layout; this is done in order to achieve a better reachability with the forefinger.

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Common configurations are:

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• "12 Bass" accordion: Fundamental Bass goes from B♭ to A (the third to eighth column in the picture above), and only has Fundamental Bass and major chords.
• "24 Bass" goes from A♭ to A, and has Fundamental Bass, major and minor chords
• "32 Bass" goes from E♭ to E, and has FB, major, minor and seventh chords
• "48 Bass" goes from E♭ to E, and has all six rows
• "72 Bass" goes from D♭ to F♯, and has all six rows
• "80 Bass" goes from C♭ to G♯, and has everything except diminished
• "96 Bass" is as 80 Bass, but with all six rows
• "120 Bass" goes from A♭♭ (i.e. low G) to A♯ — that's 20 columns — with all six rows.