The size of a watch is measured as the shortest distance across the dial side of the main plate (also called the lower or pillar plate), measured through the center. Today this measurement is made in millimeters. In Europe, until recently it was made in lignes, and the thickness of the pillar plate was measured in douzième.
In the United States, a system of pillar plate sizes was used that was based on an interval of ¹⁄30th of an inch.¹ A size 0 watch had a pillar plate 1 5⁄30 inches in diameter; a size 1, 1 6⁄30 inches, and so on. For sizes smaller than 0 (00, 000, etc), a thirtieth is subtracted.
National Bureau of Standards.
Circular 43, 2nd ed.
Washington, D.C.: U.S.G.P.O., 1921.
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Last revised: 28 December 2004.