A440 (pitch standard)
A440 or A4 (also known as the Stuttgart pitch), which has a frequency of 440 Hz, is the musical note of A above middle C and serves as a general tuning standard for musical pitch.
The International Organization for Standardization classifies it as ISO 16. Before standardization on 440 Hz, other frequencies were standardized upon. Although not universally accepted, it serves as the audio frequency reference to calibrate acoustic equipment and to tune pianos, violins, and other musical instruments.