To measure your head, the tape should be placed around the headline, where the hat fitting would touch your head (fig 1). Make sure the tape is comfortably positioned and not too tight. This is the most basic head measurement, though for bespoke millinery others will also be taken. Measurements over the top of the head from side to side (fig 2) and from back to front (fig 3) will also be taken In order to check for extra wide or long heads, both of which will throw tailored brims out of line.
courtesy of the Hat Book by Alan Couldridge