Tex and millitex:

Tex is a unit of measure for the linear mass density of fibers and is defined as the mass in grams per 1000 meters. The most commonly used unit is actually the decitex, abbreviated dtex, which is the mass in grams per 10,000 meters. When measuring objects that consist of multiple fibers the term "filament tex" is sometimes used, referring to the mass in grams per 10,000 meters of a single filament. If 1000 meters of yarn weight 10 grams the yarn is said to be 10 tex. On the other side, if 10000 meters of yarn weight 20 grams the yarn is said to be 20 dtex.