countergage

from The Century Dictionary.

• noun In carpentry, a method used to measure joints by transferring the breadth of the mortise to the place where the tenon is to be made, in order to make them fit each other.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Carp.) An adjustable gage, with double points for transferring measurements from one timber to another, as the breadth of a mortise to the place where the tenon is to be made.