For HTS Chapter 61, fiber content is determined by the chief weight of the textile fibers present. A garment with 55% cotton and 45% polyester is classified as a cotton garment. The chief-weight rule applies to the fabric itself, not to trimmings or accessories, and must be supported by lab test results or manufacturer certifications if CBP requests verification.

HTS Chapter 61 separates garments by whether they are designed to be worn next to the skin (underwear, T-shirts, singlets) versus outer garments (sweaters, cardigans, track suits). The distinction is based on the garment's design and customary use, not simply its weight or thickness, and different subheadings carry different duty rates.