Watchmakers, who always have their own way of doing things, do not count in millimetres but in "lignes" to measure the size of a "calibre" (an 18th-century word to describe a movement). This measurement is usually denoted by a triple apostrophe after the figure: hence a movement can be described as having a diameter of 11'" or 11 lignes, which is 24.8 mm.A "ligne" or "ligne parisien" is an old unit of measurement used in traditional watchmaking prior to the metric system, and is directly inherited from the French pied (foot) under the Ancien RÈgime. One line equals 2.2558 mm, rounded to 2.26 mm to speed up calculation. In the past, a foot (ë) measured 12 inches (") of 12 lignes ('") each.
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