Double-knit weight or 8-ply yarn can be used for socks, accessories, shawls, wraps and heirloom sweaters. Double-knit-weight yarn is interchangeable with sport-weight yarn, but sport-weight yarn is just a little lighter than double-knit-weight yarn.
Yarn weights are often confusing to knitters, crotcheters, weavers and other fiber artists because there used to be no uniform standards; manufacturers and reference guides would use slightly different weight definitions or requirements.
In April of 2003, the Craft Yarn Council of America worked with manufacturers and publishers to create uniform standards across disciplines. These standards have brought more consistency to the manufacturers of needles and other industry suppliers.