Brow
Measure distance from front edge of crown, around the front to the front edge on the other side.
(A to B)

Total length of headstall
This is the most important measurement. Measure from the fold where the bit connects on one side, over the crown, to where the bit connects on the other side.
(C to D)

Length of noseband - fitted length, buckled. Also see English Caveson measurements below.