piano scale
A major
A1B2C♯3D4E5F♯6G♯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
AWBWC♯HDWEWF♯WG♯HA
What is true of it
A major is A, B, C♯, D, E, F♯ and G♯, and back to A.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
On the same tonic, A melodic minor differs from it by exactly one note.
Chords made only of these notes
Found by testing every chord in the catalogue against these 7 sounds, not chosen.