Note to Self: Yarn Weights
Tex numbers indicating yarn weight are fractions. The first number indicates grams per 1 kilometer (or 1000 meters). The second number indicates the number of plies that make up the yarn. Thus, a Tex number of 50/2 tells us that the yarn is made of 2 strands that separately weigh 50 grams per 1000 meters.
I notice on my current favorite site, Colour Mart, they tend to just give the first Tex number. Seems reasonable. So, as they sell yarn (I'm making an educated guess here), 50 Tex is lace, 100 Tex is sport, 400 Tex is bulky. It seems that Tex numbers are not contingent upon fiber type.
As with most things in the world, this is metric. 1 NM yields 1000 meters of yarn per kilogram (what ever those things mean). The first number in the fraction tells us the number of plies. A 1/4 NM yarn yields a yarn that has been spun 4 times longer (than a 1 NM yarn) using 1 kilogram of yarn. Same weight, different thickness; a 1/4 NM yarn will be much thinner than a 1 NM yarn at 4000 meters per kilogram. Unlike other yarn weights, the type of fiber is irrelevant.
In this case the type of fiber must be known. What ever the fiber, "1" yields yards per pound. Of course different fibers will obviously give us different yardage amounts per pound. As with the Tex numbers the first number in the Imperial fraction tells us plies. The second number indicates how many times more the length is beyond the standard. (Remember, Standard in this case is fiber-specific). Finally, cotton seems to be reverse: count precedes plies.
In general (for wool) 2/10 is DK and 3/10 is Aran.