piano scale
G major
G1A2B3C4D5E6F♯7
The seven-note scale everything else is measured against: two whole steps, a half, three whole steps, a half.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
GWAWBHCWDWEWF♯HG
What is true of it
G major is G, A, B, C, D, E and F♯, and back to G.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
On the same tonic, G melodic minor differs from it by exactly one note.
Chords made only of these notes
CmajorCmaj7major seventhDmajorD7dominant seventhEmminorEm7minor seventhF♯dimdiminishedF♯m7♭5half-diminished seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.