I don’t play a guitar, and this may be a stupid question, but if a whistle and a guitar are playing together and the whistle is a Bb, should the guitar be playing in a different key for them to sound in tune? I’ve never played a duet with a guitar, so I need a little help. Thanks!
with any luck you can just tell the guitarist that yer in Bb 
Most of them have some idea how that works.
If the whistle player is playing in the key of Bb, (not a given, even on a Bb whistle), then the guitarist will want to be in Bb also. He may do that by playing in G, with the capo on the third fret, or by playing in A, with the capo on the first fret, or by some other means. So yes, you do want to be playing in the same key. Have fun!
In my experience, plenty of guitar players have no clue what key they are playing in, or may just have a good guess based on what the first chord of a song is (which isn’t always true or won’t always make sense), and they may have to, through trial and error of moving the capo or trying the alternative chords or both, find what finally works. I know a few informally-taught guitar players who didn’t bother with theory. I taught a few of them myself.
I’ll admit that I don’t play with guitar players that I have taught.
Yer funny!
It’s not a stupid question at all. Transposition is my favorite subject. 
The guitarist doesn’t need to know or care what key of whistle you’re using. They only care about the actual concert key of the the tune you’re playing on it. Which is a result of the original tune key, and the transposing effect of the non-D whistle.
If you know how to figure that out, tell the guitarist the key. For example, an A minor tune on a Bb whistle gives you … F minor. A trad guitarist will probably capo up 3 and play in D minor.
Otherwise, just play the key note of the tune (in this case, A) on your whistle, and let them match that note on the guitar. Tell them if it’s a major or minor tune. That’s all they should need to figure out the right capo and chord-key combination.
[Sorry, Denny, that’s not 25 words or less. I blew it.]
Or tell the guitarist to use his Bb guitar 
Right, I forgot about that solution! When your guitarist shows up with one of those roll-up cases that holds 12 different guitars in every key, your worries are over.
It works with ukeleles.
http://www.youtube.com/watch?v=9ba1e9GkI4c
Okay. I’m not a music theorist, but here goes…The tune is in Bb, but it begins on a D, which I think makes it a minor key tune. I don’t know enough about music to say what key that becomes…is it D minor? Anyway, should that affect what key the guitarist plays it in? Sorry to be so ignorant, but this is new territory for me and my go to music theorist (the church organist) is on vacation.
Thanks!
What’s the tune, Bill?
I wish I had the opportunity to use this line but I don’t. But if I ever do, I’m ready. Hilarious.
s’okay…its a talent that not all have 
if the organist is willing, have a seat and have 'em show you how to read the keyboard
and count in both directions 
with any luck it 'll start to make sense.
The first note of a tune isn’t always a very
reliable indicator of the key. What’s the key
signature in the sheet music (how many
sharps/flats)?
If you have to do trial and error, try this:
Have the guitarist put a capo on the third
fret. First the guitarist can play as if the
tune were in G (G, C, and D chord shapes)
while you play the tune. If those chords fit,
the tune is probably in Bb major.
Here are the potential keys one might play
a Bb whistle in, and the chords (relative to
capo III) that most often come up in that
key:
Bb major: G, C, D
Eb major: C, F, G
C minor: Am, Dm, E7
C dorian: Am, G, C
G minor: Em, Am, B7
G dorian: Em, D, G
F dorian: Dm, C, F
You could, in a pinch, try each of those and
see if any fit (make sure you have a capo on
3)
The tune is “I Wonder as I Wander.” A Christmas standard…
Ah, OK. It’s playable on a whistle in any of 3 tune keys without tricky fingerings: B minor, E minor, or A minor. Click on this link for downloadable sheet music of the basic melody:
X:1
T:I Wonder As I Wander
C:John Jacob Niles
M:6/8
K:Bm
“B minor”
F|BFB dcB|AFD E2F|BFB dcB|dcB B2F|
BFB dcB|AFD E2F|BFB dfB|dBF E3|]
“E minor”
[K:Em]B|eBe gfe|dBG A2B|eBe gfe|gfe e2B|
eBe gfe|dBG A2B|eBe gbe|geB A3|]
“A minor”
[K:Am]E|AEA cBA|GED D2E|AEA cBA|cBA A2E|
AEA cBA|GED D2E|AEA ceA|cAE|D3|]
Note: With the A minor version, you have to fudge the lowest note because it’s out of range. But it sounds OK anyway.
Here are the transpositions if you play these on a Bb whistle:
Fingered Key → Transposed Key / Guitarkey-Capo
Bm → Gm / Em-capo3 or Dm-capo5
Em → Cm / Am-capo3
Am → Fm / Em-capo2 or Dm-capo3
In other words, if you play the B minor melody, it will sound in the concert key of G minor. Your guitarist can play in E minor with the capo on the 3rd fret, or in D minor with the capo on the 5th fret.
Now, Bill, is your guitarist reading the chords from sheet music, or figuring them out on his/her own? That will affect how they handle the guitar key information above. Let us know!