Whistles generally play two octaves, and you can easily play in two keys on any whistle, for example on a D whistle you can play in D and G. Because many tunes go down below the key note, you will often find that the tune is easier to play in G than D on a D whistle, because D is the lowest note. This means you should also consider the alternative key of the whistle when choosing the key. C goes to F, A to D which is useful, F to Bb, G to C and so on.