The strings are made from a variety of materials. Various types of plastic strings are popular. Silk strings, typically yellow in color, are still made, despite their higher price and lower durability than modern strings; some musicians prefer them, perceiving a difference in sound quality to modern strings. The strings are tied with a half hitch to a roll of paper or cardboard, about the size of a cigarette butt, strung through the holes at the head of the , threaded through the holes at the back, tightened, and tied with a special knot. Strings can be tightened by a special machine, but often are tightened by hand, and then tied. One can tighten by pulling the string from behind, or sitting at the side of the , although the latter is much harder and requires much arm strength. Some instruments may have tuning pins (like a piano) installed, to make tuning easier.