Jac.

Jac. is a clerk's shortening of Jacobus, the Latin name behind both James and Jacob. Busy writers cut common first names down to a few letters, often raising the last letters above the line.

Because Jacobus covers both names, Jac. can be a trap. Name lists give it as James, but in German and Dutch families the same letters often stand for Jacob. Look for the name written out in an English record before you decide.