Yarn is a long continuous length of interlocked fibers, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and ropemaking.

Yarn can be, and is, spun from a wide variety of materials, including natural fibers such as animal, plant, and mineral fibers, and synthetic fibers. It was probably first made from plant fibers, but animal fibers soon followed.

The direction in which the yarn is spun is called twist. Yarns are characterized as S-twist or Z-twist according to the direction of spinning. Tightness of twist is measured in TPI (twists per inch or turns per inch). In Europe more common measurement is number of twists per 1 meter.

Two or more single yarns may be twisted together or plied to form a thicker yarn. Generally, handspun single plies are spun with a Z-twist, and plying is done with an S-twist.

The twist in the yarn has a two-fold effect:

  • The twist increases cohesion between the fibres by increasing the lateral pressure in the yarn, thus giving enough strength to the yarn;
  • Twist increases the helical angle of fibers. Due to the above effects, as the twist increases, the yarn strength increases up to a certain level, beyond which the increase in twist actually decreases the strength of staple yarn.

The continuous filament yarn also requires a small amount of twist in order to avoid the fraying of filaments and to increase abrasion resistance. However, twisting the continuous filament yarn reduces the strength of the yarn. Yarn is often ply-twisted in a direction opposite to a single yarn twist to improve evenness, strength, elongation, bulkiness, luster and abrasion resistance, and to reduce twist liveliness, hairiness and variation in strength.

Twisted yarn can be made of two, three, four, or more plies, or may be used as singles without plying.