Arabic maqams, especially those containing quarter tones. While most violins have four strings, there are violins with additional strings. Some have as many as seven strings. Seven is generally thought to be the maximum number of strings practical on a bowed string instrument; with more than seven strings, it would be impossible to play any particular inner string individually with the bow. Violins with seven strings are very rare. The extra strings on such violins typically are lower in pitch than the G-string; these strings are usually tuned to C, F, and B. If the instrument's playing length, or string length from nut to bridge, is equal to that of an ordinary full-scale violin; i.e., a bit less than , then it may be properly termed a violin. Some such instruments are somewhat longer and should be regarded as violas. Violins with five strings or more are typically used in jazz or folk music. Some custom-made instruments have extra strings which are not bowed, but which sound sympathetically, due to the vibrations of the bowed strings. Bows A violin is usually played using a bow consisting of a stick with a ribbon of horsehair strung between the tip and frog (or nut, or heel) at opposite ends. A typical violin bow may be overall, and weigh about . Viola bows may be about shorter and heavier. At the frog end, a screw adjuster tightens or loosens the hair. Just forward of the frog, a leather thumb cushion, called the grip, and winding protect the stick and provide a strong grip for the player's hand. Traditional windings are of wire (often silver or plated silver), silk, or baleen ("whalebone", now substituted by alternating strips of tan and black plastic.) Some fiberglass student bows employ a plastic sleeve as grip and winding. Bow hair traditionally comes from the tail of a grey male horse (which has predominantly white hair). Some cheaper bows use synthetic fiber. Solid rosin is rubbed onto the hair, to render it slightly sticky; when the bow is drawn across a string, the friction between them makes the string vibrate. Traditional materials for the more costly bow sticks include snakewood, and brazilwood (which is also known as Pernambuco wood). Some recent bow design innovations use carbon fiber (CodaBows) for the stick, at all levels of craftsmanship. Inexpensive bows for students are made of less costly timbers, or from fiberglass