I - ii - iii - IV - V - vi - vii° is the diatonic chords formula in any major key.
The diatonic chords of the key of G major are the list of chords you can play and will always sound good together as long as you stay in the key of G major. Those chords are only composed of notes that are in the G major scale. The diatonic chords in the key of G are "G, Am, Bm, C, D, Em, F#dim".
Triads chord are the basic chords of any key and the most common chords in rock or pop music. They are all composed of only three notes derived from the first, the third and the fifth degree of the major scale.
1
I
G
Function: Tonic
2
ii
Am
Function: Pre-dominant
3
iii
Bm
Function: Tonic
4
IV
C
Function: Pre-dominant
5
V
D
Function: Dominant
6
vi
Em
Function: Tonic
7
vii°
F♯dim
Function: Dominant
Each diatonic triad chord can be replaced by other chords to spice up your sound.
In addition to the standard chords within a key, you can incorporate other chords such as "secondary dominant chords" and "chords borrowed from modes" into your chord progressions. It's important to note that these chords may introduce notes that are not part of the key's scale. Therefore, it's essential to use them thoughtfully and with an understanding of how they can impact the overall harmony.
V7/I
V7/ii
V7/iii
V7/IV
V7/V
V7/vi
V7/vii°
Secondary dominant chords are employed to introduce tension in music, with the purpose of resolving to a chord that belongs to the key. In a given key, each scale degree chord possesses a corresponding dominant chord that, when played before it, naturally guides to the intended scale degree chord.
Chords borrowed from modes (also called modal interchange chords) are chords taken from a different musical mode than the one corresponding to the key you are in. While borrowed chords can add richness and originality to a composition they require careful handling to maintain cohesion as some combinations will work better than others.