How do they measure hands in the nfl for a quarterback?

How does the nfl measure hand size for a quarter back? For example Michael Vick has a hand size of 10 1/4 inches. Do they measure the circumference of the hand or do they do something else?

barry m:
maybe from wrist to the tip of your middle finger

Barbara S:
they use a ruler to measure the spread of the fingers from thumb to little finger and from base of palm to tip of mikkle finger.