A "Double Weft" carpet means that after a row of knots is tied, the weft thread is passed through, the loom reversed, and the weft is passed back through. The wefts are then pounded down to lock the pile. On a double weft carpet, one cannot see the warp threads. In older pieces, three wefts between rows of knots was common, and in certain tribal pieces, 5 or 6 weft threads can be seen between the rows of knots. Sometimes, in double weft carpets, the alternating wefts are often made of different thickness, with the heavy weft being drawn tight and the lighter weft inserted with less tension. This has an effect of setting alternating warps in different planes resulting in longitudinal ridges on the back of the carpet. This effect produces a carpet that feels much stronger.