The difference usually comes from size.
- Both are made of plant, animal, or manufactured fibers twisted together for strength.
- Both can be made by machines or by hand. Both are used to create fabric, by machine or by hand.
- Both can be made of a single strand—a ply—or multiple strands. The more plies, the stronger the yarn/thread.
Yarn diameters can range from small to very large. Spinners, knitters, and crocheters often classify yarn into standard yarn weights. Generally, thread is very small in diameter and has its own system of classification.
Weavers often use these terms interchangeably. Spinners usually refer to their creations as yarn unless they're spinning at an exceptionally fine diameter, known as "frog hair."