Guitar chords in the key of D major:
D Em F#m G A Bm C#dim

Chord D7 notes: D, A, C and F#. You must leave out the 6th and 5th strings.

The string 4 (D) is left open.

D dominant 7th chord's alternative names: Ddom7, (D septim).

Steps: 1-3-5-b7.
1(D), 3(F#/Gb), 5(A), b7(C).