Couching or laid stitches involve two sets of threads: the set that is being 'laid' onto the surface of the fabric and the set which attaches the laid threads. The laid threads may be heavier than the attaching thread, or they may be of a nature that does not allow them to be worked like a regular embroidery thread, such as metal threads. The stitches used to attach the laid thread may be of any nature—cross stitch, buttonhole stitch, or straight stitch—but some have specific names: