A C whistle, when fingered the same way, will play C major A Bb whistle , Bb major. And so on. This basic and simplest of scales on the whistle we can call the 1st Major Key . Or you can call it the primary key if you want. It really doesn't matter.
Every whistle can also play in other keys, with varying degrees of difficulty.
We'll focus here on what I call the 2nd Major key and the Basic Minor key. The 2nd Major key begins with the note that you play by covering the top three holes only. This is G on a D whistle. It's F on a C whistle. This turns out to be an important note on the whistle.