"No, that's NOT an accordion!"
...an answer that every melodeon
player has given at least a few hundred times in his/her
life!
Though the two are related, as the melodeon is historically the accordion ancestor.
The Viennese Cyrill Demian patented the invention of the "accordion" in 1827.
His reed instrument had five keys,
each of them giving a different note when pulling or pushing the bellows.
That is the main feature that has survived in the melodeon, not in the later
accordion models.
Demian's instrument had at the beginning a good success amid the highest social classes,
but it did not last long. Instead its compactness and agility made of it an ideal instrument
for popular music, often related to open air dancing .
The meledeon followed the paths of european emigrants to the New World.
At present the melodeons are commonly played by Italian, English, French, Irish, and Canadian (folk)
musicians, and they're related to the one-row versions played by Cajun musicians and
the one row and a half ("Due Bassi") played in Central Italy.
They're also known as "diatonic accordions" . The italian current
name of the melodeon is organetto. Organetti are then distinguished, according to the
number of buttons for the left hand, in 'due bassi', 'otto bassi' (two or eight bass
buttons) and so on.
The instrument pictured above is a typical italian craft model, Castagnari
Tommy model. Cheaper, but less
performant, are the commercial German models (as Hohner for instance).
Each of the two right-hand rows (your left) contains only the notes of one particular major
scale, which is what "diatonic" means.
In the case of the Bosio Big Band instruments, the keys are G and
C, that is presently the most common in Italy (we actually biased
the melodeon market as I explain in the BBB history page; only 15 years ago the most popular
melodeon was in A-D).
The most common configuration for Morris players seems to be D and
G. Irish musicians (like for instance Sharon Shannon) tend to play ones
that look the same, but are in keys a semitone apart, usually either B-C or
C-C# (see the Irish Squeezebox
page ). There exists as well many other
other
configurations.
For those of you with a music-theoretical bent, the way this works is
that you push in on the bellows to play any note that's in the tonic chord
of a row's key, and you pull out to play any other note; same system as
a harmonica, in other words. The left-hand buttons (your right) play chords,
of which there are only six on the instrument, related of course to the
right hand scales. For a G-C melodeon: G, D, C, F, E, and A minor.
This page is partly based on Jon Berger melodeon page and on the book by Francesco Giannatasio 'L' organetto', Bulzoni Ed., Rome 1979.