Watch Movement Measuring Systems

Although the use of the millimeter to determine watch sizes is commonplace now, the predominant system still in use up until the 1960's and even to some extent in the 1970's, was the LIGNE. The LIGNE is equal to the twelfth part of the French Inch, which is approximately 1/16th larger than our own inch. One French ligne=2.25583mm or roughly 2¼ mm. Thus a 10 ligne movement should measure 22.5mm. In inches this would be 0.8881 as the LIGNE is equal to .0888 parts of an inch.

American watch manufacturers used to indicate the dimensions of their movements, by means of their own unique "Size" gauge. The "SIZE", as a unit of measurement, had its basis in their old "0" size movement. This movement measures 1" + 5/30" in diameter. Each difference in diameter of 1/30" accounts for each size smaller or larger than the starting point of the system, ie. the "0" size watch movement. Thus a "1" size movement would be 1" + 5/30" +1/30", or 1"+ 6/30" (or 1.2") and a "6" size movement would be 1 + 5/30 + 6/30", or 1 + 11/30" (or 1.366"). Watch movements smaller than the "0" size are designated by adding zeros to the size. Thus a movement one size smaller than the "0" would be a 2/0 size movement and would measure 1" + 5/30" - 1/30", the 3/0 size would then measure 1" +5/30" - 3/30". Conversely, the "25" size watch movement would measure just 2 full inches. A very confusing system compared to the modern simple metric system!