Blog: Watch SizesCopyright © David Boettcher 2006 - 2017 all rights reserved.
I make additions and corrections to this web site frequently, but because they are buried somewhere on one of the pages the changes are not very noticeable, so I decided to create this blog section to highlight new material. Here below you will find part of one of the pages that I have either changed or added to significantly.
This particular blog page is unusual in that it isn't extracted from any other page but stands on its own as a disussion about watch sizes, in particular the sizes used to measure and characterise watch movements.
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English and American watch sizes
The photograph here shows the dial plate of an English lever watch. The initials JW are for John Wycherley of Prescot, an English pioneer of mass produced rough movements or "frames". In 1866 Wycherley set up a factory in Warrington Road, Prescot, with three floors and steam power to produce by machinery plates and other parts that were interchangeable.
Above Wycherley's initials are a set of numbers that define the essential dimensions of the movement, often called in English watchmaking the "calliper" of the movement, derived from the measurement of the diameter of the bottom (dial) plate measured by a pair of callipers. The equivalent Swiss/French term is "calibre" and literally means much the same thing, although calibre is often used to characterise the specific layout of a movement as well as its size.
The following information is based on information in Philip Priestley's "Aaron Lufkin Dennison, An Industrial Pioneer and his Legacy".
The number 12 gives the size or diameter of the movement, followed by a • and then a 0 over a 3 that gives the pillar height which determines the thickness of the movement.
This calliper size is called the "Lancashire Watch gauge system" for determining watch sizes. Size zero formed the base of the scale with a plate diameter of 1 inch plus 5/30 inches for the mounting flange or "drop", giving a base for the scale at zero of 35 thirtieths of an inch, or 1.167″. Each 1/30 inch increased in diameter increments the gauge size by one. For smaller watches, the scale counts down from zero, indicated by 2/0, 3/0 etc. A glance at the table makes it clear.
The 12 on the Wycherley movement shown in the indicates that it is 1 inch plus (12+5)/30 = 1 and 17/30 inches in diameter, or 47/30 = 1.567″.
|10/0||26/30 = 1.000||22.0||9.75|
|9/0||27/30 = 1.000||22.9||10.1|
|8/0||28/30 = 1.000||23.7||10.5|
|7/0||29/30 = 1.000||24.6||10.9|
|6/0||30/30 = 1.000||25.4||11.3|
|5/0||31/30 = 1.033||26.2||11.6|
|4/0||32/30 = 1.067||27.1||12.0|
|3/0||33/30 = 1.100||27.9||12.4|
|2/0||34/30 = 1.133||28.8||12.8|
|0||35/30 = 1.167||29.6||13.1|
|2||37/30 = 1.233||31.3||13.9|
|4||39/30 = 1.300||33.0||14.6|
|6||41/30 = 1.367||34.7||15.4|
|8||43/30 = 1.433||36.4||16.1|
|10||45/30 = 1.500||38.1||16.9|
|12||47/30 = 1.567||39.8||17.6|
|14||49/30 = 1.633||41.5||18.4|
|16||51/30 = 1.700||43.2||19.1|
|18||53/30 = 1.767||44.9||19.9|
|20||55/30 = 1.833||46.6||20.6|
|22||57/30 = 1.900||48.3||21.4|
It might be thought odd that the gauge was based on 1/30 part of an inch rather than some less unwieldy fraction, but 1/3 of an inch was the smallest Anglo-Saxon unit of length and was called a barleycorn, because it was nominally the length of a corn or grain of barley. This measurement is still the basis for shoe sizes in Britain. When early watchmakers needed to specify dimensions more closely, it was natural for them to think in terms of 1/10 of a barleycorn.
The micrometer, with its associated division of the inch into "thous" or thousandths of an inch, was not invented until the nineteenth century. It might be thought that it would be difficult to make something as precise as a watch without a means of accurate measurement, but the first watches were made by hand finishing parts so that they all fitted together in one watch, they were not interchangeable between different watches. It was the development of accurate measuring instruments that allowed the mass production of watches, by enabling parts to be made to uniform sizes and interchangeable.
The Lancashire gauge system was adopted by American watch manufacturers, no doubt because before watches started to be manufactured in America, watches and watch movements were imported from England, and the Lancashire gauge size would have been used to specify the size of the movement, and hence the name for the size of the watch.
The gauge size is the diameter of the movement, not the size of the watch. A size 18 American railroad watch for instance should have a movement 1 + (18+5)/30 = 1.767 inches diameter, but the overall diameter of the case containing will be around 2.25″.
The 0 over 3 indicates the pillar height, the distance separating the two plates of the movement. Standard pillar height was taken 1/8″ indicated as 0/0, with increments indicated above the line and decrements below in 1/144″. So 0 over 3 indicates a pillar height of 1/8″ minus 3/144″, or 18/144 - 3/144 = 15/144, that is about 0.104″ or 2.65 mm - which when I first worked it out seemed very small, but I have just put a rule to the movement of an English lever watch that was lying on my desk and the gap between the plates is 3 mm, so 2.65 mm doesn't seem out of the question.
It seems a bit strange that the conversion between inches and millimetres is a nice easy to remember number rather than having a long string of decimal places. In fact this wasn't always so. In 1930 the British Standards Institution adopted an inch of exactly 25.4 mm, the American Standards Association followed in 1933 and by the mid 1930s many countries had adopted this "industrial inch", but these were industrial rather than legal standards. In Britain and the Commonwealth countries and America the inch was legally defined as one thirty sixth of a yard. Before 1959 the British imperial yard and the American yard were not exactly the same, and consequently British and American inches were slightly different. In 1959 the "international yard" was defined as 0.9144 metres and this was adopted in Britain and Commonwealth countries and America. The American legal inch changed by -2 millionths of an inch and the UK legal inch by +1.7 millionths of an inch, so that the inch of exactly 25.4mm falls mid way between the old UK and US legal inches.
Swiss and French watch sizes
Swiss and French watch movement sizes are usually given in lignes. A ligne (pronounced line) is a douzième or 1/12 of an old French inch, which itself is 1.0657 of an English inch. So a ligne is about 2.256mm. In the context of watches a douzième usually means a twelfth part of a ligne, also sometimes called a point. This can be measured with a douzième gauge
The standardized conversion for a ligne is 2.2558291 mm (1 mm = 0.443296 ligne), which is more easily remembered as 2.256 mm. The line size is abbreviated by a triple prime ‴ in a similar way to the double prime sign ″ for an inch.
Watches were made in sizes from 3 lignes, a very small lady's baguette movement, to 20 lignes or more for pocket watches. A typical Swiss made man's trench wristwatch from the Great War would have a movement around 12‴ or 13‴ (12 or 13 lignes) that is about 27 to 29.3 mm diameter, the case being about 32 to 35mm in overall diameter.
Copyright © David Boettcher 2006 - 2017 all rights reserved. This page updated January 2016. W3CMVS.