Yarn is a textile product made by processing various textile fibers into products of a specific fineness, used for weaving fabrics, making ropes, producing threads, knitting, and embroidery, among other applications. It is categorized into short-fiber yarns and continuous filament yarns.

　　There are several ways to express the fineness of yarn, such as denier, metric count, English count, and tex (see "count"). The twist of a yarn is expressed in terms of the number of twists per meter or per inch.

　　Wool yarn and wool thread are commonly used to knit wool sweaters, wool trousers, wool vests, scarves, hats, gloves, and various clothing items for spring and autumn. In addition to providing warmth, they also serve as decorative accents.

　　There are several ways to express the fineness of yarn, including denier, metric count, English count, and tex (see “count”). The twist of a yarn is typically expressed as the number of twists per meter or per inch. Twists can be either S-twist or Z-twist. Within a certain range of twist levels, the strength of the yarn increases with increasing twist. The choice of twist direction for single yarns and plied yarns depends on the intended use of the plied yarn. Generally, single yarns and plied yarns are twisted in opposite directions—either ZS or SZ. There is an optimal ratio between the twist of the single yarn and that of the plied yarn; within this range, the strength of the plied yarn increases with further increase in twist. However, once the critical value is exceeded, the strength of the plied yarn begins to decline. The properties of the fibers and the spinning method play a decisive role in determining the performance of the yarn. During ring spinning, as the yarn is twisted, fibers undergo repeated transfers—from the inner layer to the outer layer and back again, many times over. As a result, the fibers wrap around the yarn’s axis in a helical pattern, with the radius of the helix alternatingly increasing or decreasing along the axial direction. In this process, longer fibers tend to migrate toward the center of the yarn, while shorter fibers tend to move toward the outer layer. Fine fibers tend to concentrate toward the center of the yarn, whereas coarse fibers tend to gather at the outer layer. Fibers with lower initial modulus tend to occupy the outer layer, while those with higher initial modulus tend to remain closer to the inner layer. By appropriately selecting fibers with different characteristics, it is possible to spin yarns of various styles, tailored to specific fabric applications or designed to enhance wearing comfort. Since synthetic fibers allow for flexible selection of fiber length, fineness, and cross-sectional shape, outerwear fabrics are best made from synthetic fibers that are slightly coarser and shorter in length, blended with cotton, to enhance the fuzzy texture of the yarn surface. For underwear fabrics, it is preferable to use synthetic fibers that are slightly finer and longer in length, allowing cotton fibers to remain on the outer layer of the yarn, thereby improving moisture absorption and wearing comfort. Free-end spinning methods—including air-jet spinning, electrostatic spinning, and vortex spinning—produce yarns in which fibers shift inward only minimally during twisting. Consequently, the yarn core tends to be tighter than the outer layer, resulting in a looser structure, poorer fiber straightness, weaker fiber interlocking, and lower yarn strength. However, these yarns exhibit excellent dyeability and abrasion resistance. Yarns spun using self-twisting techniques, also known as self-twisted yarns, rely on friction rollers that impart periodic, alternating false twists to the sliver. As a result, the yarn features periodic regions without any twist, leading to relatively low strength. Such yarns are generally combined into plied yarns before being woven into fabric.