Re accidentals and melodeons: it depends on the particular model of melodeon. In a 2-row D/G box, you can borrow notes from the other row (so you can play a C# in a G tune, should it arise)but you may or may not have other accidentals. My box has no accidentals, so I can't play an F natural at all. Some boxes have a selection of a few accidentals available on the bottom keys, and some have an extra half-row of accidentals, as well as extra bass buttons to give extra chords.

Players of chromatic instruments sometimes find it hard to understand why anone would devise or play an instrument that looks quite complicated but doesn't have all the notes, but most melodeon players quite enjoy playing within (and sometimes avoiding) the limitations of the instrument. If I wanted to play a chromatic instrument I'd have an accordion instead.

Each melodeon player devises ways of playing or avoiding accidentals, and in ensemble playing you can get away with it as long as someone else is playing the notes/chords. There's that G# in Da Slockit Licht, which I just omit, or there's the bit in Horses' Brawl when it goes to G minor and needs a B flat, so I just vamp open chords with no thirds for that bit.

That's probably more than you wanted to know, Les, but basically the melodeon players will sort it out for themselves. Just don't expect them to be able to find a note or chord that simply isn't on the instrument.

And Greg: yes I take your point about arpeggio-style tunes being easy to play badly. But I suppose if the player can learn at that stage to play with a light touch rather than relying on the bellows for the rhythm, maybe they'll develop a better playing style overall? And I do think arpeggio-based tunes help a beginner to figure out how the buttons work and relate to each other.

Marje