Bookbinders likely know that linen thread is classified by a two number system, such as 35/3. And most know that the second number represents the number of threads plied together, and the first number how thick or thin the thread is. But what does the first number actually refer to?
It turns out that two different systems, an English system and a Metric system that use a similar two part description of size separated by a forward slash. However, these two systems are not the same. Most thread sold by bookbinding supply companies uses the English System.
The English system (aka. Number English, Lea, NeL, Linen Count) is based on how many skeins (of 300 yards) make up one pound in weight. I *think* this means that twelve 12/1 skeins would weigh one pound, or thirty-five 35/1 skeins would weigh one pound. I’m still not sure how adding the plied threads results in the classification. Would a 35/3 thread weigh 3 pounds?
The Metric system (Nm, aka. the Japanese Gunze Count) is based on how many meters of thread weigh one gram. So I think for a 60/1 thread, 60 meters weighs one gram. It is the same as the English system in that overall, thicker and stronger threads have lower numbers.
Other thread systems include:
Tex — How many grams 1,000 meters of a thread weighs. In this case, the larger the number, the thicker the thread.
Denier — How many grams 9,000 meters of a various thread weights. Again, the larger the number, the thicker the thread. This is useful for very thin threads and microfibers.
Grist — Yards per pound. For example, a 20/1 linen is 3,000 yards long per pound. Different fibers have different weights.
I’m still not sure what system the Londonderry Linen Lacing Thread in the image above uses. It is labeled only a mysterious “#4”. I love sewing with this thread, though, since it is thick, soft, easy to untwist, tangle free without waxing, and remarkably compressible. It is possible to sew a book naturally packed with it. It consists of five plies, and is roughly equivalent to a 20/5.
If you are wondering what size thread you should use to sew a book, check out my Guide to Swell.
Finally, Colophon Book Arts is a reasonably priced, one stop shop to purchase a wide variety of sewing threads.
Cor Knops, of Knops Boekrestauratie in the Netherlands, kindly sent me these images of some antique thread he owns.
These packages contain hanks of thread, and all weight about a pound. I think the package on the left is 25/3, and on the right 12/3. So if my calculations are correct, the 12/3 should contain 1200 yards of thread, assuming a 12/1 would contain 12 – 300 yard skeins = 3600 yards. Enough thread for a lot of books in any case!