Changes

The G is a perfect fifth interval above C. In the key of C Major, G is the 5th note of the scale, and the G Major chord is thus called the ''V chord'' (that's the roman numeral five, not the letter 'v'). It's notes are the perfect 5th, major 7th, and major 9th (octave of the major 2nd) above the tonic note of the key ((G B D) for the key of C Major). ''Note: like all major chords, it consists of a root and the M3 and P5 above the root. G is the root; B is a M3 above G; D is a P5 above G.''

The V Chord is unambiguously a <u>dominant function</u> chord. This means it produces a large amount of tension and "wants" to resolve to a tonic-function chord.  The 7th note of the major scale is called the leading tone because it naturally "wants" to resolve up a half step to the tonic note. The B note in the G major chord is the leading tone of the key of C Major and wants to resolve a half-step up to C.  The root note of the V chord is also the fifth of the I chord, making its transition from the V chord to the I chord seamless. The G in the G chord becomes the 5th of the C chord (C E G).

Here's a piece of tab. Above the tab lines there's the chord names - G and C. Below that are V and I. Tab or sheet music is often marked with these roman numerals to identify the harmonic function of that part of the music. This assumes that we are in the key of C Major. If for instance, we were in the key of G Major, the markings would instead be I and IV.

''Note: This assumes that we are in the key of C Major. If for instance, we were in the key of G Major, the markings would instead be I and IV. Given that any piece of music can have multiple ways to interpret the key and even which chords are being sounded, there is no one correct way to interpret the music. However, usually pieces tend to begin or, more importantly, end on the tonic note/chord. We can use that as a starting point, then analyze the harmonic movement we get. For the above example, if we were in the key of G Major, we'd have a I - IV transition, which is considered to build tension rather than relieve it. Using the key of C Major instead we have a V - I cadence, the strongest and most "satisfying" chord progression. So it makes more sense to assume we're in the key of C Major with a V - I chord progression. That appears more likely to be the intent of the composer and how most audiences will perceive it.''