1 Weight Yarns
A CYC 1 weight is also called Super Fine, Sock, Fingering, and Baby. This weight of yarn is often used for items that need to be lightweight such as baby items, socks or shawls. Crochet hook sizes from 2.25 mm up to 3.5 mm also known as B-1, C-2, D-3, to E-4 Hooks. Knitting Needle sizes from 2.25 to 3.25 mm or sizes 1, 2, and 3. While still a thinner yarn your stitches will be less defined depending upon your yarn choice.
Source: Craft Yarn Council of America's www.YarnStandards.com