Blending cotton with synthetic fibers — most commonly polyester — produces thread with higher tensile strength, better resistance to abrasion and heat, and reduced shrinkage compared to high-cotton-content thread. These properties are particularly valued in denim, workwear, and upholstery stitching where seam durability under stress is critical. The trade-off is reduced breathability and a different dyeing behavior compared to pure cotton thread.

For textile products, US origin rules under the HTS generally apply a tariff-shift rule: the country where the thread is spun (i.e., where the yarn is formed from fiber) is typically the country of origin, not the country where the fiber was grown or the country of final packaging. For NAFTA/USMCA preference claims involving Canada or Mexico, the applicable textile-specific rules of origin in the agreement govern. A licensed customs broker should be consulted to confirm the applicable rule for the specific blend and sourcing scenario.