Thread containing 85% or more cotton by weight falls under 520411 (non-retail) or 520420 (retail), while thread with less than 85% cotton is classified under 520419 (non-retail). The cotton percentage by weight must be determined at the fiber composition level, and supplier fiber content certificates are the standard supporting document.

Sewing thread is specifically engineered for stitching — it is typically highly twisted, finished with lubricants or glazing, and wound on spools or cones in forms suited for sewing machines or hand sewing. Other cotton yarn intended for weaving, knitting, or embroidery is classified under the cotton yarn headings rather than here, even if it could physically be used for sewing.