Whoever it was that figured out how to construct the Stradella bass was a genius. If you ever have cause to look at the innards of the bass side of the accordion you will find an amazing set of levers and rods which open the appropriate valve (for bass note buttons) or valves (for chord buttons) depending on which buttons are pressed.
Only 3 notes in a chord
The chord buttons select the appropriate three notes from a single chromatic octave of notes depending on which chord button you hold down. This means that any chord you get is often an inversion of the expected chord. Of course, by using the couplers on the bass side, the order of the sounding of the notes can be varied still further. Our ear sorts out how the chord ought to sound.
The critical point here is that only 3 notes are sounded*. This is perfectly alright for a major chord (C major is C, E and G) or a minor chord (C minor is C, Eflat and G) but falls down when you have chords that call for 4 notes, such as C 7 (C, E, G and Bflat) or C dim7 (C, Eflat, Gflat and A). In these cases, the decision was taken that the valve not to be opened would be the one associated with the 5th (for C7 it would be G and for C dim7 it would be Gflat). I believe the thought is that our mind will "hear" the missing note as it is an over-tone of one of the notes being played in many cases (not in the case of diminished chords, however).
*Tom Bell-Richards got in touch to point out that some older accordions do play 4 note chords for 7ths. He suggests that the reason for the reduction to only 3 note chords was to allow for the chord combining such as that discussed in these pages. In fact, my MIDI system generates 4 note 7ths even though the reeds are only sounding 3 notes.
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