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.
Addendium:
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!
Peachey Conservation 
I have bundles of old Linen in skeins which I need to photograph for you. it is amazing to sew with and was [i was told] spun around a wire [?] and was designed especially for the binding trade. This post is fascinating though I tend to use my fingers to tell me what to sew with. Still, it’s very good stuff.
Love to see the photos, Tim!
I love this article! Thank you for sharing this information, and so clearly put, too!
Thanks!
I believe the first number in the English system represents the thickness of the strand, not the whole thread. If you use all three-strand thread then the thickness numbers will also represent the thickness of the whole thread. If you use a mixture of two-, three-, and four-strand thread sizes, then putting them in order becomes much more complex; to say nothing of the complications introduced by tightness of twist and amount of dressing in different brands.
Yes, that is what I was trying to say, though perhaps with the wrong terminology, by using a hypothetical “35/1” thread to represent one strand of, say, a “35/3” thread. Good point about tightness and actual size. One brand of 25/3 might be closer to another brand’s 25/3.
I’m looking for some spools of kinda flat linen string that is very strong that I use to wrap my longbow limbs with after I use high temp glue sticks. At least about 3 mm wide if flat, or about 2mm wide if rounded.
I’m afraid this is out of my area of experience!
Found 1 lb cone of vintage #6 linen cord. Wondering yardage this might be. Purpose is to use it as rug warp. Any idea? Judy
No, different crafts seems to have developed different thread nomenclatures. Have you checked modern suppliers of linen for rugs? You might be able to estimate.
The system for cord is entirely different from that used for thread. Furthermore, two examples of what binders call “cord” can be entirely different products to the maker; many years ago, accustomed to the thickness of “flax seaming twine,” I bought Clarkson cord that was far thicker than what I expected. And the systems used in different countries differ, for instance French linen thread numbers are completely different from British thread numbers.
Go from what you have, not from theory. If you have a high-precision scale weigh a yard or two of the cord and weigh the full cop. Despite the unknown weight of the cardboard core, this should give you a guesstimate order-of-magnitude yardage. With luck you will have either ten times as much as you need, or only a tenth as much. If your need and your guesstimate are close, then you’re screwed.