In music, A 440 is known as the a above middle C, and vibrates at 440 cycles per second (440 hertz). Because it is just above the bass clef, A 440 is a good note for tuning musical instruments, since almost all instruments have 440 hz within their native range. When a symphony orchestra performs, the concertmaster will direct the oboe to sound the A, and the other instruments will tune to it. A 440 is usually sounded twice before a performance, and the instruments play a sort of 'humming' sound as they compare their tune to that of the oboe. Once in tune, the concertmaster will sit and the conductor will take the stage so the music may begin.