Cotton has long been used for backing threads because it makes a light, firm foundation which is less stretchy than wool. Both warps and wefts of most Persian rugs are cotton.
In the American machine industry, domestic run-of-the-mill cotton is used, usually in combination with jute, paper, or co-jute (cotton and jute). In some of the better fabrics all the backing yarns are cotton.
There are many all-cotton rugs on the market today in both flat and pile weaves.
They are light and washable, but soil much more quickly than woolen rugs. A cotton pile lacks the resiliency of wool so is more prone to get limp and flat.