Why are crochet hooks sized differently?

Crochet hooks are sized differently to create various stitch sizes and to match different types of yarn. The desired crochet hook size and number of stitches that hook makes in an inch is usually printed on the label of a skein of yarn.

Steel crochet hooks are specifically made to create lace with thread. These hooks are numbered from 14, the smallest size, to 00, the largest size. The remaining sizes of crochet hooks are designated by a lettering system by which B is the smallest hook at 2.25 millimeters in diameter and Q is the largest at 16 millimeters. These hooks are commonly made of aluminum, plastic or bamboo.