In string instruments, which of these is used to cause a sound?

In music, a bow is a tensioned stick which has hair (usually horse-tail hair) coated in rosin (to facilitate friction) affixed to it. It is moved across some part of a musical instrument to cause vibration, which the instrument emits as sound. The vast majority of bows are used with string instruments, such as the violin, although some bows are used with musical saws and other bowed idiophones.

A bow consists of a specially shaped stick with other material forming a ribbon stretched between its ends, which is used to stroke the string and create sound. Different musical cultures have adopted various designs for the bow. For instance, in some bows a single cord is stretched between the ends of the stick. In the Western tradition of bow making—bows for the instruments of the violin and viol families—a hank of horsehair is normally employed.

The manufacture of bows is considered a demanding craft, and well-made bows command high prices. Part of the bow maker's skill is the ability to choose high quality material for the stick. Historically, Western bows have been made of pernambuco wood from Brazil. However, pernambuco is now an endangered species whose export is regulated by international treaty, so makers are currently adopting other materials.