John: "The exception proves the rule. Franglo and free-bass accordeon are exceptions to a pretty consistent rule! "
But we haven´t found that "rule" yet have we...?? It obviously can not be as simple as ..." a concertina - ... has one note per button"
OK, here we go!
As a rule ...
- Concertinas are hand-held or supported on the knee(s). Any attachments are to the player's hands, not his body
- Concertinas have a bellows cross-section that is nearly or exactly a regular polygon (English: hexagonal, octagonal, occasionally 12-sided, seldom square; German: approximately square, latterly hexagonal)
- Concertinas have a continuous tonal space, usable for either melody or harmony, distributed over both ends (usually with an overlap)
- Concertinas, if monosonoric, have one note per button; if bisonoric, one note per button/bellows direction, except for so-called drones (multiple reeds may sound when a button is pressed, but they are tuned in unison or octave, so they are functionally perceived as one note)
I could go on, but I think these "rules" are pretty definitive, and allow exceptions to be identified.
Note that there are two major categories of Concertina: mono-sonoric and bi-sonoric. Each category has a number of variants. Mono-sonoric concertinas are the EC and the various Duet systems, all sharing the same construction principles.
Bi-sonoric concertinas are the German, Anglo-German, Anglo-Chromatic, Carlsfelder, Chemnitzer, Bandoneon. Some are of German-type construction, some English-style. They share a central pad of 20 buttons arranged in two rows according to the Richter Scale, though the additional buttons are different in each case.
Yet these are all concertinas according to the "rules" above.
Accordeons, too, belong to two major categories: chromatic and diatonic. Here, too, button arrangements vary: C-Griff, B-Griff or piano on the descant side; Stradella or various bi-sonoric arrangements in the bass - or free-bass. Even in the latter case, we still clearly have an accordeon variant!
BTW, when I embarked last summer on my cruise in a 60-foot steam puffer, sharing a roughly 2 x 2 x 2-metre cabin with another passenger, and after a plane and train journey to get there, I was glad that my musical equipment consisted of a standard-sized, 30-button Anglo concertina!