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
dtex × 0.9 = denier. Example: 20 dtex = 18 den. Equivalently dtex × 9/10.
den = dtex × 0.9
Example: 1 dtex = 0.9 den
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.