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?
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.