Yarn units are a type of measurement used to quantify the amount of yarn in a given package. Skeins, clews, and plies are all types of yarn units. A skein is a unit of yarn that is wound into a loose twist and is usually 6 letters long. A clew is a unit of yarn that is wound into a tight twist and is usually 5 letters long. A ply is a unit of yarn that is made up of multiple strands of yarn twisted together and is also usually 5 letters long.