G Major
0 - 4 - 7Root + major 3rd + perfect 5th
About G Major
G major is the chord that makes a guitar sound like a guitar. Three fingers, six strings, and the two lowest notes are G and B — the root and third of the chord sitting right at the bottom where the instrument is heaviest. Nothing else in first position rings quite like it.
The notes and the two common shapes
G, B, D — root, major third, perfect fifth. Two fingerings dominate. The three-finger version leaves the B string open, giving a sound that is bright and slightly hollow. The four-finger version frets the B string at the third fret, doubling the D and making the chord thicker; it also puts your hand one move away from C.
Which one to use is not a matter of correctness. The three-finger shape suits strumming, the four-finger shape suits songs that move between G and C, because the ring and little fingers stay planted.
Why G is a guitar key
The key of G major has one sharp, F♯, and every chord in it falls under the hand: G, Am, Bm, C, D, Em. Four of those six are open shapes. Compare with E♭ major, where five of six need a barre — the same music, several times the effort.
This is why so many folk and country songs live in G. The key was not chosen for its sound; it was chosen because the instrument allows it.
Where it shows up
As tonic in its own key, as the IV of D major and as the V of C major. That last role is the most common single use of the chord in Western music: G is what resolves to C, and the pull is strong enough that a song can end on it and still feel unfinished.
Put a flat seventh on it — G7 — and the pull sharpens further. That is the difference between a G that could go anywhere and a G that is clearly on its way home.