This article is from the Concertina FAQ, by Chris Timson firstname.lastname@example.org with numerous contributions by others.
In fact there are several systems of duet concertina, each as separate
from each other as an anglo is from an English, but all set out to cure
the same perceived problem: how to give an accompaniment to a melody
without going schizoid. The answer is the same in all cases: put the low
notes on the left hand side, and the high notes on the right hand side.
The player can then play the melody on the right hand, with an
accompaniment on the left, thus the name of "Duet".
The main duet systems are:
MacCann: the key layout looks fairly illogical, but it was apparently
designed for speed rather than logic and there are certainly some
very fine players around! Fairly easy to get one. Sometimes can be
huge instruments with up to 80 buttons and the range of a piano!
Crane: also known as Triumph by the Salvation Army who used it a
lot. Much more logical system, I'm told, and again some very good
players around. Fairly easy to obtain one.
Jeffries: designed for anglo players to convert to. Has a "home key"
such as G and is apparently difficult to play chromatically, thus
players tend not to stray far from the home key. Somewhat rarer
than the first two.
Hayden: a modern system. Much the most logical, easy to learn and
straightforward duet system, with some ingenious characteristics
that make key transposition easy, but quite hard to get because it's
modern. I once asked Brian Hayden how many Hayden duets there were
in the world, and after some thought he said "Oh, about 60". However
this situation is changing markedly for the better, as Stagi have
started making accordion-reeded Haydens, a Russian bayan maker has
made prototypes and intends to go into production with a potentially
excellent instrument (the fabled Haydenovskaya), and now that The
Button Box (see section 8) have started making anglos and Englishes
they intend to return to their long-held plan to makeHaydens. Other-
wise the only option is to get one built to order by C & R Dipper or