Direct Count from Mass and Length — Tex, Decitex and Denier

The direct system is a division and nothing more: weigh a known length of yarn, divide, and you have its linear density. The three names — tex, decitex, denier — differ only in the reference length each weighs against. Tex is the grams in 1000 m, decitex the grams in 10 000 m, denier the grams in 9000 m. That is the entire content of the system, and it is why denier = 9 × tex and dtex = 10 × tex are exact by definition rather than conversions anyone had to measure.

Why three? Denier got there first, from the silk trade, where the reference skein was 9000 m and a denier was a small coin used as the reference mass. One denier happens to sit close to the linear density of a natural silk filament, which is what made it convenient. It survives in hosiery — 15 denier stockings — and in ripstop nylon and ballistic cloth, where 210 D and 1680 D are ordinary specifications. Tex arrived with the metric system's tidying-up and is codified in ISO 1144. Decitex exists because tex turned out too coarse for single filaments: a fine polyester filament is 1.2 dtex, which as 0.12 tex would have been a decimal nobody wanted on a specification sheet.

The measurement is fussier than the arithmetic. Two things routinely go wrong with it. The first is moisture. Textile fibres are hygroscopic — cotton at 65% relative humidity and 20 °C carries something like 7 to 8% of its dry mass in absorbed water, viscose considerably more, polyester almost none — so a specimen weighed straight out of a hot dryer reads several per cent fine, and one weighed on a humid afternoon reads coarse. This is why testing standards specify a conditioned atmosphere and why a laboratory keeps one. The second is tension. Yarn is extensible, so the length that comes off a wrap reel depends on how hard the reel pulled: too much tension and you have measured a longer, finer yarn than the one in the cone. The skein method for linear density is ISO 2060, and it specifies both.

One point of vocabulary that saves arguments. Count is a general word covering both systems; linear density is specifically the direct measure; denier, tex and dtex are units of it. A specification that says "count 150" with no letters is ambiguous and worth a phone call — 150 denier is a fine filament yarn, 150 tex is a heavy spun one, and they differ by a factor of nine.