Calibers and Lines – What’s that?

Caliber originally refers to the inner diameter of a pipe. So what is the caliber of this pipe here?

[Quelle: © Hans Hillewaert / , via Wikimedia Commons]
[Source: © Hans Hillewaert / , via Wikimedia Commons]

None at all! The pipes with caliber specifications were, of course, the pipes of firearms. We’ll come back to the calibers in a moment…

Initially, movements for pocket watches were mostly round, and the diameter of the movement was used to differentiate between them. The original unit of measurement was the Parisian line, or ligne, abbreviated with three quotation marks ´´´, an old measure of length based on the Parisian foot:

1 Parisian line = 1/12 Parisian inch = 1/144 Parisian foot

One line (1´´´), “Ligne” in French, corresponds to 2.256 mm.
Incidentally, the Parisian Foot and the units derived from it were reference units used throughout Europe in the 17th and 18th centuries.

Here is a size comparison of some common diameters of wristwatch movements:


Back to the calibers! A movement with a diameter of 10 1/2´´´ was simply called caliber 10 1/2´´´ in technical jargon. In other words, an abbreviation for “a movement with a diameter of 10 1/2 lines“.

In Germany, the metric system was introduced in 1872. However, most watchmakers and movement manufacturers do not care about this. They still like to use the line as a unit of measurement today.
In the past, you could also buy calipers that displayed lines as well as centimeters:

As there were many different movement manufacturers, the manufacturer had to be specified in addition to the diameter in order to identify a movement, e.g. caliber AS 10 1/2´´´ (AS = Adolph Schild).

Manufacturers soon began to build different movements of the same diameter. These had, for example, different escapements, such as a swiss lever or cylinder escapement, or simply different bridge shapes to appeal to different buyers. Here is an example of a technically identical movement with different bridge shapes:

[Quelle: Classification Horlogère 1936]
[Source: Classification Horlogère 1936]
As a result, each manufacturer came up with its own logic to differentiate the movements. The most common was to simply use numbers, but there are also letters, combinations of numbers and letters and even plain text names.

These designations were soon used as caliber designations instead of the diameter, e.g. caliber AS 313 (see picture above). One of the best-known current movements, for example, has the designation ETA 2824-2. The actual information on the diameter was therefore usually lost in the new designations.

So we note:

  • In the past: caliber = diameter of the movement in Parisian lines
  • Today: Caliber = designation for the movement type (series)


However, we still need to clarify a few open questions about lines and caliber.

  1. How do you specify the dimensions of a form movement in lines, i.e. a movement that is not round?

For round movements, the diameter is indicated; for form movements, the length of both sides is indicated. The value of the side on which the winding stem is located is given first:


This movement here, for example, has 5 ¾ x 7 ½ lines:


2. Can you rely on the manufacturer’s diameter specifications?

Unfortunately, no. For example, very few movements with 10 1/2 lignes have a diameter of exactly 23.69 mm. Large movements were often stated to be somewhat larger and small movements were usually smaller in reality than on the manufacturer’s data sheet. These specifications were therefore rather useless for the construction of watch cases.

3. Can you simply interchange different movements of the same diameter?

In 99.9 percent of cases, unfortunately not. Even if the diameter is exactly the same, there are other parameters that usually do not match:

  • The movement height
  • The installation height of the winding stem
  • The position of the dial feet on the movement
  • The fit (hole diameter) of the hands

4. Are there any other units of measurement for the diameter of movements other than lines or millimeters?

Sure, otherwise it would be boring. In the USA, and to some extent in the rest of the world, the Lancashire Gauge size was used for a long time.

5. What are Lépine (open face) and Savonnette (hunter) calibers?

These “caliber” specifications have absolutely nothing to do with the diameter and are not manufacturer-specific. They refer to two different construction types of pocket watch movements.

In the “classic” pocket watch without a cover on the glass, also known as a Lépine, or open face, the small second is normally in line with the crown and the 12 is located by the crown. The Savonnette, or hunter, is usually operated with the crown at 3 o’clock when the cover is opened. It is then more practical if the small second is perpendicular to the crown.

For this reason, many movements are available in both a Lépine and a Savonnette version. The picture shows the Unitas 6498 (Savonnette) on the left and the Unitas 6497 (Lépine) on the right:


The red lines show the position of the winding stem (in the picture at the top) in relation to the small second (red circle).

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