Thread Sizes
Joy Beeson near Albany, NY, USA
#70 tatting thread is equivalent to #35 sewing thread; #80 is equal to#40.
The "50" on sewing thread is short for 50/3, which means that three "yarns"have been plied together, and that it takes fifty hanks of such yarn toweigh a pound. Or maybe it's fifty skeins to the ounce, or apermutation. It's been a long time & I've forgotten the details. At any rate, you can see why bigger numbers mean smaller yarn -- the thinner the yarn, the more yards it takes to make a pound.
(The word "yarn" was later adopted to distinguish the thick, soft thread used for knitting -- perhaps because it looks like the yarns used for making rope.)
All sewing thread was three-ply when the numbers were established, so the "/3" was left out. Then when six-ply threads came along, 100/6 thread was labled "50" because #50 was what the customers were used to buying, and to this day sewing thread is numbered according to the size of cotton yarn you would three-ply to get the same diameter -- even though few threads are cotton, and all decent cotton sewing thread is six-cord. From the beginning, good crochet cotton was six-ply, and the best crochet cotton still is. This led to the habit of leaving off the "/6" -- so crochet and tatting threads have numbers twice as large as the numbers on corresponding sewing threads.
Different fibers were wound in hanks (or is that skeins?) of different length, and they also have different densities -- so a 100/6 linen thread would *not* be equivalent to #50 sewing thread.
To confuse the matter further, worsteds write the number of plies on top and the size of the plies on the bottom -- 3/12, for example, is a fine three-ply wool knitting yarn, and 4/8 is a four-ply yarn that used to becalled "knitting worsted." (Hence, our current use of "worsted weight" and "4-ply" for yarns about this size.) Woolens use a different hank length but I *think* that sizes are indicated by the same kind of fractions as worsteds. I seldom buy woolen spun wool, so I haven't looked into it.
Since the number of yarns is on top, and the number of hanks in a pound is on the bottom, a larger worsted fraction indicates a thicker yarn. (the first "yarn" is not the same word as the second "yarn" -- one of the reasons I try to avoid discussing this subject.)
Silk threads are indicated by weight in denier. "Denier" means grams per 9000 meters of yarn, and bigger numbers mean coarser thread. Silk fabrics are measured in "mumy" or "mummy", abbreviated "mm" and all I know aboutthat system is that a bigger number is a thicker fabric. My cataloguses"sheer" only in descriptions of 8mm and down, but I can read bold type through white 19.5mm satin.
Sewing silk is lettered: Size A is ordinary sewing thread, finer than#50cotton, but stronger. Size B is heavy-duty (I've never seen any on spools for home-sewing machines, but I once bought a cone of size B at a garage sale), and Size D is buttonhole twist.
(I collected every spool of silk thread I saw during the years when it was hard to find, because silk is stronger than cotton, but doesn't cut fibers the way nylon and polyester do. So my rugged wool and leather athletic clothes are darned with dainty pastel colors.)
In six-cord cotton sewing thread, #50 is the ordinary weight, #60 is used for sheer fabrics, and #40 for coarse work. There used to be a "heavy duty" thread that, if I recall correctly, was between #50 and #40 in thickness.#8 thread is good for sewing on buttons and making loops or eyes for hooks.(I prefer a nylon thread of about the same thickness as #8 for making eyes, since nylon stands up to abrasion.)
Cotton thread with fewer than six plies is fit only for embroidery and basting -- though I recently used two-ply thread to sew a break-away seam when mending a pocket that is likely to catch on the same doorknob again.
Sometimes six-cord thread is made by twisting three two-ply threads together. If a thread separates into only three smaller threads, look at one of them with a strong magnifying glass before relegating the spool to the basting bin.