Thrown silk yarn (5004) is made by twisting continuous filaments reeled directly from cocoons, producing a smooth, lustrous yarn with high tensile strength. Spun silk yarn (5005) is made by carding and spinning short fibers recovered from silk waste, resulting in a slightly duller, softer yarn with more texture. Spun silk is generally less expensive and is used where the full luster of thrown silk is not required.

Classification of blended yarns depends on the fiber composition rules in Section XI of the HTS. If silk is the predominant fiber by weight, the yarn may still fall under the silk chapter, but a blend where another fiber predominates would typically be classified under that fiber's chapter. The specific blend ratio and the applicable chapter notes govern the outcome.