A440

A440 refers to the musical pitch A above middle C (A4) with a frequency of 440 Hz in modern equal temperament. It is commonly used as the standard reference tone for tuning musical instruments.

In practice, many orchestras, bands, and instrument manufacturers calibrate their instruments to A440. Electronic tuning devices commonly base their reference on A4 at 440 Hz, and pianos are often tuned so that the A above middle C resonates at that frequency.

Historically, concert pitch varied by region and era. The adoption of A4 = 440 Hz as a standard pitch was established in international standards, notably ISO standards that designate this frequency as the reference for A4. Despite its widespread use, some ensembles and periods of repertoire employ alternative pitches, such as 435 Hz or slightly higher or lower values, depending on performance practice or historical authorization.

The standard has practical implications for instrument design, construction, and performance. While A440 is the prevalent reference in contemporary concert settings, it is not universal, and performers may adjust tuning for specific works, venues, or historical interpretations.