The dulcimer comes in various sizes, identified by the number of strings that cross each of the bridges. A 15/14, for example, has 15 strings crossing the treble bridge and 14 crossing the bass bridge, and can span three octaves. The strings of a hammered dulcimer are usually found in pairs, two strings for each note (though some instruments have three or four strings per note). Each set of strings is tuned in unison and is called a course. As with a piano, the purpose of using multiple strings per course is to make the instrument louder, although as the courses are rarely in perfect unison, a chorus effect usually results like a mandolin. A hammered dulcimer, like an autoharp, harp, or piano, requires a tuning wrench for tuning, since the dulcimer's strings are wound around tuning pins with square heads. (Ordinarily, 5 mm "zither pins" are used, similar to, but smaller in diameter than piano tuning pins, which come in various sizes ranging upwards from "1/0" or 7 mm.)