Yarn is a long continuous length of interlocked fibres, used in sewing, crocheting, knitting, weaving, embroidery, ropemaking, and the production of textiles.

Yarn can be made of a number of natural or synthetic materials, and comes in a variety of colors and thicknesses (referred to as "weights").

[1] The human production of yarn is known to have existed since the Stone Age and earlier prehistory, with ancient fiber materials developing from animal hides, to reeds, to early fabrics.

Cotton, wool, and silk were the first materials for yarn, and textile trade contributed immensely to the ancient global economy.

The most common plant fiber is cotton, which is typically spun into fine yarn for mechanical weaving or knitting into cloth.

The protein fiber of silk is composed mainly of fibroin and is produced by the larvae of the moth Bombyx mori.

[10] Other animal fibers used include alpaca, angora, mohair, llama, cashmere, and silk.

[9] More rarely, yarn may be spun from camel, yak, possum, musk ox, vicuña, cat, dog, wolf, rabbit, bison, or chinchilla hair, as well as turkey or ostrich feathers.

These strands are drawn (stretched), annealed (hardened), and cured to obtain properties desirable for later processing.

Tow is a continuous "rope" of fibers consisting of many filaments loosely joined side-to-side.

It is often made from the remainder fabric of clothing manufacture, and therefore is considered a recycled and eco-friendly product.

[13] In general, natural fibers tend to require more careful handling than synthetics because they can shrink, felt, stain, shed, fade, stretch, wrinkle, or be eaten by moths more readily, unless special treatments such as mercerization or super washing are performed to strengthen, fix color, or otherwise enhance the fiber's own properties.

Some types of protein yarns (i.e., hair, silk, feathers) may feel irritating to some people, causing contact dermatitis, hives, or wheezing.

According to a study reviewing the evidence of wool as an allergen conducted by Acta Dermato-Venereologica,[15] contemporary superfine or ultrafine Merino wool with their reduced fibre diameters do not provoke itch, are well tolerated and in fact benefit eczema management.

[15] Further studies suggest that known allergens applied during textile processing are minimally present in wool garments today given current industry practices and are unlikely to lead to allergic reactions.

Pilling is a function of fiber content, spinning method, twist, contiguous staple length, and fabric construction.

Yarns combining synthetic and natural fibers inherit the properties of each parent, according to the proportional composition.

Synthetics are added to lower cost, increase durability, add unusual color or visual effects, provide machine washability and stain resistance, reduce heat retention, or lighten garment weight.

Spun yarn is made by twisting staple fibres together to make a cohesive thread, or "single".

Depending on the direction of this final twist, the yarn will have either s-twist (the threads appear to go "up" to the left) or z-twist (to the right).

It was originally applied to synthetic fibers to reduce transparency, slipperiness and increase warmth, absorbency and makes the yarn more opaque.

[29] Yarn quantities for handcrafts are usually measured and sold by weight in ounces (oz) or grams (g).

Some companies also primarily measure in ounces with common sizes being three-ounce, four-ounce, six-ounce, and eight-ounce skeins.

Gauge, known in the UK as tension, is a measurement of how many stitches and rows are produced per inch or per cm on a specified size of knitting needle or crochet hook.

The proposed standardization uses a four-by-four inch/ten-by-ten cm knitted stockinette or single crocheted square, with the resultant number of stitches across and rows high made by the suggested tools on the label to determine the gauge.

There are many different ways in which yarn is wound, including hanks, skeins, donut balls, cakes, and cones.