The raised 5th assumption.

What would you assume if a raised 5th degree of the key signature's corresponding major key, appears in the first few measures of a tonal piece?

Examples:
Key Signature: 0 flats, 0 sharps. Key Signature for C major. The raised 5th is G#.
Key Signature: 2 sharps. Key Signature for D major. The raised 5th is A#.
Key Signature: 3 flats . Key Signature for Eb major. The raised 5th is B natural.

In fact there's quite a few assumptions you can make. But based on the style and period of the piece (of course composer too) we can prioritize as follows:

1. We are on the relative minor key.
Explanation. The relative minor shares the same key signature as its relative major, but still needs it 7th degree raised to create the harmonic tension of the V (dominant)
Verification: Checking the first and last notes of the piece, for the relative minor chord, will probably tell us if this is true.

2. A strong vi chord is being targeted.
Explanation: In this case we would be in a major key (C). The raised 5th of that key (G#) is the 7th degree of the vi chord (Aminor) which is the 3rd of the V7/vi (see Why is there an F# on C major? post)
Verification. Checking the first and last notes of the piece, for the major chord(C) and verifiying that the chord used under the raised 5th (G#) is the V7/vi (E7) and that the chord after that is the vi (Aminor).

3. The note is part of a line cliche.
Explanation. It is very common to used a harmonic progression with alterations of the I chord.
I - I+ - I6 (sometimes keeps going - Ib7 - I7..)
Verification: Checking the chord below the raised 5th degree is still the I, and the raised 5th is a passing tone between 5 and 6.


mDecks

Why is there an F# on C major?

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???

mDecks