I think I have a better understanding of this than I used to have but if I haven't (or indeed have) could someone who knows put me right?

We play and sing with a friend who is a very nice whistle player and it's a really nice addition to lots of the songs. The only two major keys we don't currently sing a song in are Ab and Db; and the only minor keys we don't sing songs in are Abm, Bbm, Db, Eb, F and Gb.

My friend has (to my knowledge) D, G, A and C whistles and a couple of recorders.

Sometimes when we play she'll say "ah, a whistle would be nice on that" and we then try and work out which one and usually get it wrong until we find one that works. She can play half holed notes if needed but doesn't want to play loads of them!

So I thought I'd try and understand and this is what I have come to...

The D whistle should be happy to play in D and G major and Bm and Em.
The G whistle should be happy in G and C major and Em and Am.
The A should cover A and D major and F#m and Bm.
The C should cover C and F major and Am and Dm.

So each whistle does tonic and subdominant and their two respective minors?

Am I missing anything else out?

I'll give you an example. We sing Water is Wide in E (E or a B whistle would work if my understanding is corret - but she has neither). But I reckon that it might be able to be played on an A whistle as it only has one instance of a D# in it. Would this be normal or practical? ie To start the scale on the second hole rather than the 6th or 3rd hole which I believe (now) is what it is all based on? It just opens up other possibilities occasionally.

I realise there is a second bit to this which is whether the tune itself works on the particular whistle - ie if the range of the tune doesn't fit the whistle without having to go very high. And that brings in whether it sounds fitting for the song.

But at the moment I'm just trying to check that I have understood what I have been looking at. And whether it applies in the real world!

I hope some of that is clear. Any help gratefully accepted

You might also ask - why all the weird and wonderful keys when we could nudge them up or down a semitone. Two singers in harmony and sometimes the odd semitone makes it sound better to accommodate either a high or low note comfortably.