14.4Leading-tone diminished chord

The note one semitone below the tonic is the leading tone.

In C:

B → C

Build diminished chord on B:

B D F

So:

B° → C

In G:

leading tone = F♯

F♯° → G

In D:

leading tone = C♯

C♯° → D

General idea:

vii° → I