Deciphering Watch Crystal Sizes: The X/16 and X/8 Ligne Systems

Watch crystals are a critical component of timepieces, offering both a clear view of the watch’s face and protection for the delicate hands and dial. The size of these crystals is typically indicated on stickers adhered to the glass crystals when new. These stickers usually display at least two measurements, one on top and one below, leading to confusion regarding the correct measurement to use when selecting a crystal.

The Ligne: A Brief History

The Ligne is a traditional unit of measurement used in watchmaking. It originated in France and was formally defined during the French Revolution as 1/12th of a ‘Pouce‘, the French equivalent of an inch. This made it equal to 2.2558291 millimeters, and this measurement has been used in the watchmaking industry ever since.

The term ‘Ligne‘ comes from the Latin ‘linea‘, meaning ‘line‘. In its original context, it referred to one of the lines that were used in the typefaces of early printing presses. However, it was adopted by the watchmaking industry due to its suitability for measuring the components used in timepieces.

The X/16 Ligne System

The X/16 Ligne system, as the name suggests, divides the Ligne into 16 parts. This means that each increment in this system is equivalent to about 0.141 mm. This system is often used for its precision, as it allows for more accurate measurements of watch crystal sizes.

On the stickers attached to the watch crystals, the X/16 measurement is usually displayed on top. For example, a sticker might read “13 3/16,” indicating a diameter of 13 and 3/16 Lignes.

The X/16 system is generally considered the standard method for measuring watch crystals due to its precision and wide adoption. For this reason, the older X/8 system is largely ignored when selecting a crystal. However, understanding the X/8 system can still be useful in understanding the alternative measurements used for crystals.

The X/8 Ligne System

The X/8 Ligne system, on the other hand, divides the Ligne into 8 parts. Each increment in this system is equivalent to about 0.282 mm. This system is based on an older Ligne system variation with a different zero point. Size 0 in the X/8 system is equivalent to 10 Lignes, and each full-size increment corresponds to half a Ligne.

On the stickers attached to the watch crystals, the X/8 measurement is usually displayed below the X/16 measurement. For example, a sticker might read “13 3/16” on top and “14 5/8” below, indicating a diameter of 14 and 5/8 Lignes according to the X/8 system.

Crystal Height

In addition to the diameter, the stickers on watch crystals often also display a third number corresponding to the crystal’s height. This height is usually expressed as a whole number between 1 and 8, with 1 indicating a taller crystal and 8 indicating a shorter one. The crystals with a lower height and a greater number are typically used for hunting cases, where the crystal needs to fit with a low profile inside the case when it’s closed.

Conclusion

Understanding the measurements on the stickers attached to watch crystals can be a bit tricky, especially considering the variations in the Ligne system. However, with a bit of knowledge about the origins and uses of the Ligne, as well as the X/16 Ligne system, it becomes much easier to decipher these measurements and understand the size and height of the watch crystal you’re dealing with. Although the X/8 system is largely ignored in modern watchmaking, it can still be useful when dealing with older timepieces.

Crystal Conversion Table: X/16 Ligne vs. X/8 Ligne vs. Millimeters