Everything in music is referenced to the white keys

For example C# G# is an interval of 5th, we think of the white notes C, G. We count the white notes or keys. C to G is 5.


Other example. Moonlight Sonata - Beethoven begins with bass C and 1,3,5 on the right hand. Ie C,E,G. Actually they are played in a different order, G, C, E. The key is C sharp minor most of the notes or keys are black. But still even the names are of the white keys. We say C and sharp because it is sharp. C#. And we can think of 1.3,5 as G,C,E but it is G#,C#,E.


And we always write the white notes on the score. But we know from the beginning by special notation at the leftmost part which ones are sharp or flat.