The term rug yarn can also be applied generically to any yarn used to make rugs. Used this way, the term can include yarns with different characteristics from those described above. Tapestry yarn, a 4-ply tightly twisted yarn, is often used in rug making, as is Rya yarn, a 2-ply yarn with a ropelike twist, and Lopi, a traditional Icelandic 1-ply wool yarn. The important factor, if you're using a pattern, is to match the knitting gauge recommended.
Rug yarn also comes in several novelty styles, though less variety is found in yarns labeled "rug yarn" than in yarns meant for, say, socks and sweaters. One novelty form of rug yarn is glitter yarn, and flake and nub yarns are also available. Color is another area in which rug yarn has a bit less variety than some other weight yarns. Natural, undyed rug yarn is available, along with a wide variety of colors, including some heathered colors. Some natural rug yarn may accept dyes.