During the sessions many times you can hear, that you don’t know the title of the played tune and you want to know the key ;). Whistle is a simple diatonic instrument, each one can played in several keys only. E.g. D whistle, without half holes, you can play also G, Em, Bm. Other keys (see table below) require a bit of practice, but it is not impossible.
As a person, who regularly plays with vocalists, I have to be prepared for every eventuality, so from the table below I have almost a full set (I do not have F # and G #), but as you can see from this table, I have the possibility to play in virtually any key. My standard equipment for sessions is a whistle’s set and a phone application: Song Key Finder :).