Sew-in interlining is a type of fabric used to add structure and support to garments. It is typically made from cotton, polyester, or a blend of both, and is sewn directly into the fabric layers of clothing. This technique helps improve the shape and durability of items like jackets, collars, and cuffs. Unlike fusible interlining, which uses heat to bond with the fabric, sew-in interlining requires stitching to attach it. This method allows for more flexibility and is often preferred for delicate fabrics or tailored garments, ensuring a polished finish without compromising the material's integrity.