In “sewing thread 100D”, “100D” refers to the unit indicating the thickness of the sewing thread. Here, “D” stands for Denier, a commonly used fineness index in the textile industry.Specifically, Denier (D) is defined as: the weight (in grams) of a fiber or yarn that is 9000 meters long. Therefore, “100D” means that 9000 meters of this sewing thread weighs 100 grams.