The bow’s purpose is to generate sound from a string instrument by being pulled along the strings or striking against them. A bow consists of a stick of hardwood ‑‑ traditionally pernambuco, brazilwood or amourette, although in modern times the stick might also be made of composite materials such as carbon fibre (see carbon bow). A bundle of hair from a horse’s tail is attached to the stick, and the frog is used to tighten the hair to the tension needed to play. Up until the Baroque period, bows had a convex curve (i.e. bending outward), and their frogs were sometimes locked in position or offered only minimal ways to make adjustments. Modern bows are concave (bent inward) and become more narrow towards the top. This design feature is based on very complicated rules which in part determine the major difference in quality amongst bows. On a modern bow, the frog is tightened to whatever degree is desired by means of a screw at the end of the stick.