The three branches of the woodwind family have different sources of sound. Vibrations begin when air is blown across the top of an instrument, across a single reed, or across two reeds. Reeds are small pieces of cane. A single reed is clamped to a mouthpiece at the top of the instrument and vibrates against the mouthpiece when air is blown between the reed and the mouthpiece. Two reeds tied together are commonly known as a double reed. This double reed fits into a tube at the top of the instrument and vibrates when air is forced between the two reeds.