A440 (pitch standard)

${ \hide Staff.TimeSignature \set Staff.midiInstrument = #"grand piano" a'1 \set Staff.midiInstrument = #"violin" a'1 }$

A440, piano and violin

A440 (also known as Stuttgart pitch^[1]) 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. While other frequencies have been (and occasionally still are) used to tune the first A above middle C, A440 is now commonly used as a reference frequency to calibrate acoustic equipment and to tune pianos, violins, and other musical instruments.

^ Apel, Willi (1969). Harvard Dictionary of Music. Harvard University Press. ISBN 9780674375017. Retrieved 2023-01-09 – via Google Books.