Dupioni silk (meaning "double") is produced when two or more silkworms spin their cocoons too closely together.  These slightly tangled cocoons produce filament that is rough, uneven and not as strong as cultivated silk.  It is usually reeled and used to make fine or heavy yarns with pronounced irregular slubs at random intervals.  Dupioni silk usually comes from cultivated, rather than wild, silkworms because their living quarters are more crowded, increasing their chances of tangling the cocoons.  The irregular, crosswise ribs of dupioni are formed by filling yarns that have occasional soft, thick lumps.  The lumps, or slubs, add texture and visual interest to the fabric.