plastic.) Some student bows, particularly the ones made of solid fiberglass, substitute a plastic sleeve for grip and winding. The stick was traditionally made of pernambuco - the heartwood of the brazilwood tree - but due to overharvesting and near extinction at its original source, other woods and materials, such as ironwood or graphite, are more commonly used. Some student bows are made of fiberglass. Recent innovations have allowed carbon-fiber to be used as a material for the stick at all levels of craftsmanship. The hair of the bow traditionally comes from the tail of a white male horse, although some cheaper bows use synthetic fiber. The hair must be rubbed with rosin occasionally so it will grip the strings and cause them to vibrate; new or unrosined bow hair simply slides and produces no sound. Bow hair is regularly replaced when the ribbon becomes skimpy or unbalanced from hair breakage or bow bug damage or the violinist feels the hair has "lost its grip." Strings Violins have four strings, usually tuned to G, D, A, and E. The strings run from a tailpiece attached to the base, across a wooden bridge, continue towards the neck of the instrument running parallel to the fingerboard, and connect to the pegbox located at the very top of the violin. They are wound around four tuning pegs that are mounted sideways through holes in the pegbox. The bridge helps to hold the strings in place, while the pegs maintain the tension necessary to produce vibration. Strings were first made of sheep's intestines (called "catgut"), stretched, dried and twisted. Contrary to popular belief, violin strings were never made of cat's intestines. Gut strings are used in modern and "period" music, though in recent years the "baroque" historically accurate performance violinists seem to use them more often than those who play later period music or baroque music in a "modern" style. Gut strings are made by a number of specialty string makers as well as some large stringmaking companies. In the 19th century (and even earlier though not yet prevalent) metal windings were developed for the lower-pitched gut strings. Wound strings avoid the flabby sound of a light-gauge string at low tension. Heavier plain-gut strings at a suitable tension are inconvenient to play and difficult to fit into the pegbox. There are many claims made that gut strings are difficult to keep in tune. In