The bawu is a free-reed aerophone with a cylindrical bore, made of a tube of bamboo closed off at one end by a natural node. Near the closed end, a small square hole is cut and a thin reed of bronze or copper is fastened, with a low plastic or bone mouthpiece around it. This reed is essentially a very thin sheet of metal with a long and narrow isosceles triangle cut into it, which is bent slightly outwards at rest. When the instrument is blown, this thin triangle moves back and forth rapidly through the space left in the metal sheet from which it was cut, like a swinging door. This vibration sets the air column in the instrument in rapid periodic motion, creating sound. The mouth does not contact the reed. Seven or eight finger-holes are positioned 90 degrees out of line with the reed, though this is adjustable in the common two-piece instruments provided with a metal tenon.