Crochet manufacturing conventions treat thread and yarn quite differently: manufacturers designate different sizing scales for thread and yarn. Thread is generally packaged on spools instead of skeins or hanks and is offered for sale in a separate section from ordinary yarns or threads. Crochet hooks for use with thread are also sized according to a different scale from yarn hooks. Thread hooks are also manufactured differently from yarn hooks: modern yarn hooks are usually aluminum or plastic, while thread hooks are made of steel and have smaller hook heads and shorter shanks.
The division between yarn and thread is somewhat arbitrary: crochet thread at its thickest is similar in diameter and behavior to fine cotton yarn. The largest sizes of thread crochet hooks overlap with the smallest sizes of yarn crochet hooks.
Crochet thread comes in sizes from 3 to 100, although historically it came in much finer sizes, down to 200. Diameter is inversely proportional to number, so size 3 is nearly as thick as yarn and size 100 is as fine as sewing thread. Thread may also be categorized by number of plies and size 10 thread is known as bedspread weight. Smaller sizes (40 and up) are rarely used anymore.