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The Lancashire gauge for determining watch sizes is of English origin and is the standard commonly used by American watch manufacturers. By this system 1 5/30 inches was taken as a basing figure and was called naught (0) size. Every 1/30 of an inch added increased the size one number, every subtraction of 1/30 of an inch decreased the size one number. The size of a watch is determined by its width through the center at the narrowest part of the dial side of the lower plate. The table below shows Lancashire gauge watch sizes in terms of fractions of inches, decimal inches, lignes and millimeters.

You can also view the entire American Watch Size chart that is available at horologist.com using the following link.

American Watch Sizes By Antique Time.

Here is the cut-and-paste link.

http://www.horologist.com/watch_sizes.htm