FAQ: What is a Ligne in Watchmaking?

 In FAQ, Watch Talk

Ligne is one of those terms that you may have seen before but not knowing exactly what it means. I certainly had never heard of it before I became interested in watches and watchmaking.

When I did read about it, I didn’t know exactly what it was. I figured it was watchmaker’s jargon and mainly used to try and impress people. It turns out I was wrong.

It’s a term that comes from a rich history of Swiss and French watchmaking and it’s actually still useful today.

So, what is a ligne and why is it important?

What is a Ligne?

1 foot (‘) is equal to 12 inches (”) and 1 inch consists of 12 lignes (””). 1 ligne is equal to 2.2558 mm which you can round up to 2.26 mm.

The ligne is an oldfashioned unit of length that’s still being used today, mainly in watchmaking. It was part of a system that several countries used before the metric system.

That’s why the symbol of the ligne is the triple prime and sometimes it’s also abbreviated by the letter L. Keep in mind that these are old Parisian feet and inches. They’re different from our modern variations.

Why is it important?

Watchmakers Still Use It

Watchmakers have been using the ligne for a very long time. It was even the predominant unit of measurement for movements until the early 1970s.

It’s very important to be aware of the size in ligne of the movement you’re working on, especially if you’re working on older pocket watches.

Watchmakers still use it today because of tradition and it’s more convenient to list a movement as 10½ L than 23.69 mm, for example.

What is a ligne in watchmaking?

As you can see, watchmakers list the size in ligne in the technical documents. You can download these technical documents for ETA movements here

Identifying movements

Sometimes it’s essential information to identify a movement.

Usually, there’s some info on the movement itself. Either on the top plate (underneath the balance wheel or on the wheel train bridge) or on the bottom plate (usually on the larger surface near the bottom barrel arbor hole).

If you can’t find any info on the movement, you need to identify it with the help of a catalog like Bestfit.

Bestfit lists the movements on the shape, then on the size in ligne, and then it shows all the different keyless works for that size, especially the set lever spring. So, if you don’t know the size in ligne, there’s no way that you can successfully identify the movement using this method.

Tools & Lubrication

There are certain tools that are only suitable for movements of a certain size in ligne. Adjustable movement holders are a good example.

The Bergeon 4039 is adjustable from 3 ¾ to 11 L and the Bergeon 4040 from 8 ¾ to 19 L.

The same goes for lubricants. For example, the British Horological Institute has a guide for selecting lubricants and they advise Moebius 9010 for the pivot jewel for the escape wheel in movements up to 8¼ L and Moebius 9020 for movements over 13 L. In between, you can use either 9010 or 9020.

Personally, I prefer to use 9010 for movements up to 13 L.

So, there you have it. The ligne is an important part in the history of horology and it’s still being used today. To be honest, it’s something that you’ll mainly encounter if you’re into technical specifications or hobby watchmaking but it never hurts to learn something new. You never know when it might come in handy.

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