If you’ve ever gone to hear an orchestra play, you know that the performance begins only after a session of tuning. Tuning means adjusting the pitch of an instrument. The pitch of a sound depends on its frequency—the number of vibrations per second that produce the sound. For example, plucking a cello string might cause it to vibrate back and forth 200 times each second. You hear that sound as a low pitch of 200 cycles per second, or 200 hertz. An orchestra tunes itself to a very particular frequency, usually 440 hertz, a note known as A 440. The note is played by the oboist, and the rest of the orchestra tunes their instruments to match it. The oboe leads the tuning because of all the instruments, it is least affected by humidity or other weather conditions.