The đàn bầu, also called độc huyền cầm is a Vietnamese stringed instrument, in the form of a monochord (one-string) zither.

A monochord, also known as sonometer, is an ancient musical and scientific laboratory instrument, involving one string. The term monochord is sometimes used as the class-name for any musical stringed instrument having only one string and a stick shaped body, also known as musical bows. According to the Hornbostel–Sachs system, string bows are bar zithers (311.1) while monochords are traditionally board zithers (314). The "harmonical canon", or monochord is, at its least, "merely a string having a board under it of exactly the same length, upon which may be delineated the points at which the string must be stopped to give certain notes," allowing comparison.