Weaving yarns, finest to thickest, to scale
What does "8/2" actually look like next to "2/8", and is a rug wool really that much thicker than a towel cotton? Here are the common weaving yarns on one card, finest at the top, thickest at the bottom, all drawn at the same scale.
How to read the card
Each row is a wraps-per-inch card: the yarn wound side by side across the same inch, no gaps, no overlap. The number on the right is the wraps per inch, marked M where it was measured and E where it was estimated. Each strand is drawn 1/WPI of an inch wide. The scale bar at the foot is an inch, with a centimetre under it.
The wraps are measured counts from yarn sellers' charts where they could be found. Where they couldn't, they're scaled from a measured yarn of the same fibre (thickness goes with the square root of the yardage). Setts are shown alongside but don't set the width: a sett carries weaving habits as well as thickness.
Wraps counted by hand vary from one wrapper to the next, so rows bracketed together are about the same. 20/2 and 16/2 linen come out close, 38 and 36 wraps: 16/2 has fewer yards to the pound, so it's the thicker of the two.
What the numbers mean
Yards per pound tells you which yarn is finer within one fibre. Across fibres it misleads: wool is bulkier than cotton or linen at the same yardage, so a 2/18 merino wraps thicker than an 8/2 cotton. Metres per kilo is the same measure, about twice the number.
The counts work from a standard: a size-1 single is 840 yards to the pound in cotton, 300 in linen, 560 in worsted wool and 256 in woollen. Multiply by the count for a single, divide by the plies. So 8/2 cotton is 8 × 840 ÷ 2 = 3,360 yards a pound (Schacht's figure; the Williamsburg table says 3,000). 20/2 linen is 20 × 300 ÷ 2 = 3,000, and 16/2 linen 2,400.
US suppliers usually write cotton and linen size first and wool plies first, but it varies: 2/8 worsted is 8 × 560 ÷ 2 = 2,240. Gist Yarn notes that in other countries the numbers are sometimes reversed, but the smaller one is usually the plies. Silk is often sold in the metric count: 60/2 means two plies of a 60 Nm single, about 30 km per kilo, which is 14,880 yards a pound. Real yarns also run a little off the rule (8/2 linen is 1,040 on Schacht's chart, not 1,200), so trust the maker's yardage.
Why sett follows thickness
Ends per inch is just how many of those strands sit side by side in the reed. For plain weave, every end crosses the weft at every pick, so it takes about half the wraps per inch, leaving room for the weft to pass. Handwoven's Ask Madelyn wraps a half inch for that reason: the count is usually the plain-weave sett. Twill crosses less often and packs closer: Schacht's chart gives 8/2 cotton 20-24 for tabby and 24 for twill. But weavers' habits differ too, which is why the setts on the card don't always follow the wraps.
Rugs turn the rule round. For a weft-faced rug the warp is set much more open than plain weave, so the weft packs in and covers it. Handwoven gives carpet warp's plain-weave sett as about 15, and for boundweave 8 or 10 ends an inch. A warp-faced rep goes the other way, set very dense to hide the weft.
Put your own yarn on the card
Wind it round a ruler for one inch: firm and even, no stretching, the strands touching but not overlapping.
Count the wraps. Find the row on the card with that count, and you have its neighbours.
Halve the count for a plain-weave sett, then sample and wash before you warp a whole project. Wool fulls in the wash, and the numbers are only where you start.
Sources
https://www.handweavers.co.uk/silk-602nm.ir
https://www.handweavers.co.uk/yarns-silk.irc
https://weavingyarn.co.uk/count-chart
https://gistyarn.com/blogs/how-to-weave/understanding-weaving-yarn-sizes
https://handwovenmagazine.com/ask-madelyn-yarns-and-setts/
https://woolery.com/products/harrisville-shetland-wool-yarn
https://woolery.com/products/harrisville-highland-wool-yarn
https://woolery.com/blogs/the-woolery-blog/ask-nancy-yarn-sizes
https://www.cottoninc.com/wp-content/uploads/2017/12/TRI-1014-Yarn-Numbering-Systems.pdf
https://schachtspindle.com/pages/sett-chart
https://www.williamsburgspinweave.org/ProjectPlanningforWeavers.pdf
https://janestaffordtextiles.com/products/8-2-cotton-1
https://janestaffordtextiles.com/products/harrisville-shetland
https://www.yarn.com/products/valley-yarns-16-2-linen
https://www.camillavalleyfarm.com/knit/harrisvillewool.htm
https://handwovenmagazine.com/determining-warp-sett-for-boundweave/
https://handwovenmagazine.com/yarns-and-setts-for-rep-weave/