Wool is a textile fibre produced by the shearing of sheep. The breed of the sheep, where it was raised, and how the fibre was treated after shearing can all affect the final characteristics of wool. While some is quite harsh and rough, other wool can be luxuriously soft and squishy. As a fibre, wool is very good at trapping heat, and is one of the few fibres that stays warm even when wet.

Merino sheep create a wool with a long, smooth fibre that is lightweight, warm, and soft. Like all wool, it has excellent breathability and does an excellent job regulating temperature in garments. It is a very popular type of wool to use for hand knitting yarn, and is a great choice for socks and garments worn close to the skin.

Mulesing is a controversial farming practice used, predominantly in Australia, to prevent the parasidic infection flystrike. While mulesing prevents infection, it is a very painful procedure for the animal to endure, and more humane alternatives exist. This practice is banned in many areas of the world. If a yarn is marked as mulesing free, that means that no animals were treated this way during the production of the yarn.