C major is the key with no sharps or flats.
So why is there an f# on almost every piece in C major?
Well, the answer is in the piece’s harmonic progression.
For a piece to be in C major, we need the C major chord to be the I. The I only feel as a I if there’s a V7 before it (G7) and if we have the 7th degree moving to that 1st degree (B –>C). That G7 makes the C major stronger when it resolves to it.
But how do you make the V7 (G7) stronger? You use the V7 of that V7 (V7/V). In this case you use the V7 of G, and that’s D7.
D7 has an F# in the triad (the 3rd of D) (F#–> G works as the 7th degree going to 1st)
And there’s your F# on C. Every time (almost every time, we should say) you see an F# on a C major piece, the harmonic progression is going through the V7/V (that’s D7). So look for the D, the A and the C in the rest of the harmony and you’ll probably find them (D7 = D F# A C).
Ok… Why is there also a C# on C major then???