If you are looking for the best quality cotton rug warp around... this Swedish spun cotton warp is probably it. Long staple length plus a very high twist equals high strength. Try to break it with your hands, and you'll probably hurt your hands!
So what do the numbers mean? Without getting too technical and asking you to use a lot of math, the first number is the 'grist' (or thickness) of each strand that makes up the yarn, and the second number is how many of those strands there are. The '12/6' yarn has this smallest overall grist (thickness) so conversely it has more yards in a pound. And '12/18' is approximately 3 times thicker than the '12/6'. Sorry, we don't yet have a sample card for the Viking (Swedish) cotton rug warp. It is all 'natural' color as shown in the image.
"...cotton seine twine. I usually select from three options for my own work: 12/6 for 10 to 12 epi; 12/12 or 12/15 for 8 epi; and 12/18 for 6 epi. The size of each of those is different, with 12/6 being the smallest and 12/18 the largest, as the sett indicates..."