denier
Linear mass densityexact by definition
The denier is the mass in grams of 9000 metres of filament, exactly 10⁻⁶/9 kg/m, or about 1.111 × 10⁻⁷. It is the traditional silk and synthetic filament unit and survives in hosiery, fishing line, parachute fabric and ballistic nylon: 15 denier stockings, 210 denier ripstop, 1680 denier ballistic cloth.
Watch out: Denier and dtex differ by a factor of 0.9, so 1 den is 1.111 dtex. The two are near enough that specification sheets sometimes list a number without saying which, and 10 percent is exactly the size of error nobody catches.
| 1 den | 1.1111111e-07 kg/m |
The 9000 metres comes from the old silk trade, where the standard skein was 9000 m and a denier coin was the reference mass. One denier is close to the linear density of a natural silk filament, which is what made it a convenient unit in the first place.