To measure a cap diameter, measure the cap from one side of the inner wall of the cap to the opposite inner wall. Simply place the cap on the lip of the left side of the  Cap Measurer with the notch flush against the cap inner wall. Read your measurement at the opposite cap inner wall. Measurements are shown in millimeters. This is also commonly called the "T" dimension.To measure a bottle neck opening diameter, measure the diameter of the outermost threads by placing the opening of the bottle firmly against the notch on the right side of the cap measurer. Read to the outer edge of the opening on the opposite side of the neck opening.To determine the thread finish, verify how many times the threads pass one another to determine the finish. For example, a bottle 1.5 thread turns has a 410 neck finish. This is also called the "H" dimension. Refer to the Thread Finish Guide on the back of the cap measurer or use the following chart to view common thread finishes.