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

Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or other lubricants to withstand the stresses involved in sewing. Embroidery threads are yarns specifically designed for needlework.

Yarn may be used undyed, or may be coloured with natural or artificial dyes. Most yarns have a single uniform hue, but there is also a wide selection of variegated yarns:

bailloo yarn

Heathered or tweed: yarn with flecks of different coloured fibre
Ombre: variegated yarn with light and dark shades of a single hue
Multicolored: variegated yarn with two or more distinct hues (a “parrot colourway” might have green, yellow and red)
Self-striping: yarn dyed with lengths of colour that will automatically create stripes in a knitted or crocheted object
Marled: yarn made from strands of different-coloured yarn twisted together, sometimes in closely related hues

A comparison of yarn weights (thicknesses): the top skein is aran weight, suitable for knitting a thick sweater or hat. The manufacturer’s recommended knitting gauge appears on the label: 5 to 7 stitches per inch using size 4.5 to 5.1 mm needles. The bottom skein is sock weight, specifically for knitting socks. Recommended gauge: 8 to 10 stitches per inch, using size 3.6 to 4.2 mm needles.

Spool of all purpose sewing thread, closeup shows texture of 2‑ply, Z‑twist, mercerized cotton with polyester core.

Yarn drying after being dyed in the early American tradition, at Conner Prairie living history museum.
Yarn quantities for handcrafts are usually measured and sold by weight in ounces or grams. Common sizes include 25 g, 50 g, and 100 g skeins. Some companies also primarily measure in ounces with common sizes being three-ounce, four-ounce, six-ounce, and eight-ounce skeins. Textile measurements are taken at a standard temperature and humidity, because fibers can absorb moisture from the air. The actual length of the yarn contained in a ball or skein can vary due to the inherent heaviness of the fibre and the thickness of the strand; for instance, a 50 g skein of lace weight mohair may contain several hundred metres, while a 50 g skein of bulky wool may contain only 60 metres.

There are several thicknesses of craft yarn, also referred to as weight. This is not to be confused with the measurement and/or weight listed above. The Craft Yarn Council of America is making an effort to promote a standardized industry system for measuring this, numbering the weights from 1 (finest) to 6 (heaviest).[11] Some of the names for the various weights of yarn from finest to thickest are called lace, fingering, sport, double-knit (or DK), worsted, aran (or heavy worsted), bulky, and super-bulky. This naming convention is more descriptive than precise; fibre artists disagree about where on the continuum each lies, and the precise relationships between the sizes.

Another measurement of yarn weight, often used by weavers, is wraps per inch (WPI). The yarn is wrapped snugly around a ruler and the number of wraps that fit in an inch are counted.

Labels on yarn for handicrafts often include information on gauge, known in the UK as tension, which 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 or crocheted square, with the resultant number of stitches across and rows high made by the suggested tools on the label to determine the gauge.

In Europe, textile engineers often use the unit tex, which is the weight in grams of a kilometre of yarn, or decitex, which is a finer measurement corresponding to the weight in grams of 10 km of yarn. Many other units have been used over time by different industries.