Slub cotton is cotton which contains slight lumps and imperfections, often left in the fabric on purpose or created deliberately by knotting or twisting the fabric as it is woven or spun. Slubs can also be created by weaving with fibers of uneven width. The overall effect is a fabric with a rough and uneven texture.
Cotton is not the only material in which slubs are sometimes deliberately left in or encouraged in the final fabric. Natural linen usually has slubs in it due to the natural unevenness of the flax fibers from which it is made. Butcher linen is a slub-filled fabric used for aprons and tablecloths, and butcher cotton is a type of slub cotton sometimes used in casual suits or dresses. Tweed is a wool-based fabric filled with slubs. In addition, many varieties of silk fabrics contain slubs, including dopioni silk, pongee, shantung, silk noil, Thai silk and tussah.
High fashion t-shirts and sweaters are often created using slub cotton. While the t-shirts produced this way are not suitable for use as graphic tees, which have messages or images printed on them, the fabric is more highly textured and often prized by designers for its casual, rough look. In addition, slub denim made of cotton is sometimes used in high-end distressed jeans.