What it is
Decitex (dtex) is linear density: 1 dtex = 0.1 tex = 0.9 denier. 1 tex = 1 g / 1000 m. Not button ligne, not fabric gsm.
How to use
denier / 0.9 = dtex. Example: 18 den = 20 dtex. Equivalently denier × 10/9.
dtex = den × 1.111111111
Example: 1 den = 1.111111111 dtex
Common uses
Sewing thread, embroidery thread, and filament specs. Larger number is thicker. Stockings still often use denier.
Conversion tips
10 dtex = 1 tex = 9 den. Do not mix with ligne or gsm.