In the section of Blackston's commentries on the laws of England., in the section on the king's prerogative to fix standard weights and measures, I read:
Thus, under king Richard I, in his parliament holden at Westminster, A.D. 1197, it was ordained that there shall be only one weight and one measure throughout the kingdom . . . These original standards were called pondus regis, and mensura domini regis; and are directed by a variety of subsequent statutes to be kept in the exchequer, and all weights and measures to be made conformable thereto. But, as sir Edward Coke observes, though this hath so often by authority of parliament been enacted, yet it could never be effected; so forcible is custom with the multitude, when it hath gotten an head.
Why "an head"?

