Pluck a guitar string and it vibrates backwards and forwards, producing disturbances in the surrounding air. The pitch of the sound we hear is closely related to the frequency of these vibrations. Frequency is the number of complete oscillations occurring each second and is measured in hertz. Higher-frequency vibrations are generally heard as higher-pitched notes. A guitar string is fixed at both ends. Waves travelling along the string reflect at these fixed points. At particular frequencies, the reflected and incoming waves combine to form standing-wave patterns. The lowest-frequency standing wave is called the fundamental. Higher-frequency patterns are called harmonics. The frequencies produced by a string depend on its length, tension and mass per unit length. Pressing the string against a fret shortens the section that is free to vibrate, increasing its fundamental frequency and producing a higher note. Turning a tuning peg changes the tension. Increasing the tension generally increases the frequency. The vibrating string itself moves relatively little air. Vibrations are transferred through the bridge to the body of the guitar, which helps move more air and produce a louder sound. Music therefore depends on wave physics. Length, tension, frequency, resonance and standing waves combine to turn the vibration of a simple string into a recognisable musical note.