The stocking frame invented in 1589 for silk became viable when in 1759, Jedediah Strutt introduced an attachment for the frame which produced what became known as the Derby Rib, that produced a knit and purl stitch. This allowed stockings to be manufactured in silk and later in cotton. In 1768, Hammond modified the stocking frame to weave weft-knitted openworks or nets by crossing over the loops, using a mobile tickler bar- this led in 1781 to Thomas Frost's square net. Cotton had been too coarse for lace, but by 1805 Houldsworths of Manchester were producing reliable 300 count cotton thread.