What I mean is, if I look at a piece of music written in D, and want to play it on a G whistle. Do I still play the D with all holes covered, , or would I play it as xxoooo?
Aodhan
D would be xxoooo. xxxxxx on a G whistle is G. If you played “D” as xxxxxx, and the other notes accordingly, you would actually be playing the tune in the key of G.
If you’re playing a piece written in D, and you play it as on a D whistle (all holes covered to play D), then it comes out in G on a G whistle. The whistle itself transposes the key.
The converse is, if you play a tune written in a different key, then you must transpose mentally. For a tune written in C, you must cover all the holes for C, three holes for a written F, etc.
I find tunes written in C not all that tough to read, anything else is pretty bad (except for possibly E, I dunno if I’ve tried that).
Are you playing the tune in G, or are you playing in D on a G whistle? If in G, then you play as if in D (but the key will be G); if you’re playing in D, then yes, you would play as if playing in A on a D whistle. 
N, hope this helps
Aodhan? Here’s my bit 'bout transposing instruments.
Foreword: the “concert” whistle is in D. Everything is supposed fingered for the D whistle. Anything else is a transposing instrument. You grab it, make “as if” it was in D…
Chapter 1) You read D, play D (xxx xxx)
If on a Bb tinwhistle, play “as if on a D”, you hear Bb = you have transposed in Bb
If on a G whistle, don’t you care–just play “as if on a D”, thou hark G, fer thou didst transpose in G
Chapter 2) Suppose the score is in G (1 # at the signature) and you read a C;
On a D whistle, you play C (oxx ooo) hear C = you’re playing in G scale (normal “G” tune)
On a F whistle, you play “as if” C (still oxx ooo) and hear Eb = you have transposed in Bb scale.
Chapter 3) etc. (“Wizz” went the shell…) *
Appendix 1 : the high whistle is already a transposing instrument; you read D, play D–what d’you hear ? D ok, but it’s one octave higher than written–check with the piano the D right of the “keylock” C… So let’s correct the Foreword : everything’s written for a Low D!
Appendix 2 : most “non-concert pitch” woods and brasswinds are transposing instruments, where everything is written for the “generic” instrument.
- Just for the readers of Raymond Queneau
Zoob, thanks for obscuring this topic completely. I don’t think anyone else could have done it as completely.
Aodhan. This thread is terminally ill. Run a search for “transposing”, “keys”, some such terms.
Just try and explain better. I’ll learn :roll:
Btw, it’s a challenge : do try and explain what are “transposing instruments” in a simpler, thorough way.
Or at least post a link to the relevant definitive previous post. I don’t think your indications (?) for a search help much…
ok ok OK!
To put it in a concise and simple manner so as to avoid confusion:
You play a tune written in D on a D whistle. Yes? Ok. You play the same tune, the same way, on a G whistle. (ie. You’re playing the G whistle as if you were playing a D whistle) Thus, the tune that you are playing on the G whistle has been transposed to the key of G.
Make sense everyone?
Well, I didn’t mean to step on your toes. I guess I am just a bit surprised that Aodhan who has been around a while hasn’t seen one of the previous threads on this topic.
I didn’t look very carefully and missed many others that I seem to remember. Again, sorry to have sounded grumpy. 
That’s because Aodhan had a complete mental breakdown of function, and forgot that there WAS a search function. 
Aodhan
Aodhan: half an our in the… COMFY CHAIR! 
As tired as I am right now, I’m afraid 1/2 hour in the comfy chair, would quickly become a 3 hour serious nap. 
Saw an odd kiddie show on PBS the other day, “The Big Comfy Couch”. Is the comfy chair in the same family? 
Aodhan