Topic summary
A440 (pitch standard)
A440 (also known as Stuttgart pitch) is the musical pitch corresponding to an audio frequency of 440 Hz, which serves as a tuning standard for the musical note of A above middle C, or A4 in scientific pitch notation. It is standardized by the International Organization for Standardization as ISO 16. Other frequencies have been, and sometimes still are, used to tune to this note, but A440 is the reference frequency for calibrating acoustic equipment and tuning musical instruments.