piano scale
D major
D1E2F♯3G4A5B6C♯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
DWEWF♯HGWAWBWC♯HD
What is true of it
D major is D, E, F♯, G, A, B and C♯, and back to D.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
On the same tonic, D melodic minor differs from it by exactly one note.
Chords made only of these notes
C♯dimdiminishedC♯m7♭5half-diminished seventhDmajorDmaj7major seventhEmminorEm7minor seventhF♯mminorF♯m7minor seventh
Found by testing every chord in the catalogue against these 7 sounds, not chosen.