In a "box" context Stradella is (loosely) "same both way" bass buttons with at the most basic, tonic note and major chords for each note, then as instruments have more buttons, rows for "counter bass" note and minor, seventh, and diminished chords. Each set of buttons is a musical interval of a fifth from the next.
More precisely to my mind, Stradella is a system that uses ingenious linkages to generate all those posibilities from the same sets of bass and chord reeds, usually 12 for the 12 degrees of the chromatic scale. (A 120 bass doesn't necessarily have any more reeds than a 48 bass, just more linkages.)
It's a bit like having a keyboard with just two octaves of notes with an ingenious mechanical player. When you press the button for a given chord, the mechanism chooses the correct notes for that chord.
The big difference from "melodeon bass" is that with a melodeon (most of the time, there are exceptions,) each button has it's own reeds and those reeds are not used by any other button. With Stradella the same reeds are shared by all the buttons.
Simple!
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Here's a pic of the mechanism on a Hohner. The pins on the shaft attached to each button engage with pins on each rod, which turn the rod to lift the relevant pallets.
Two sets of rods, 12 each for bass and chords.
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