A-440 or A440 reffers to the note A, tuned to 440 Hz. A-440 is the standard tuning pitch for most Western music and it is also known as the Stuttgart pitch.
Before the introduction of the standard 440 Hz, many countries use the French standard since the 1860 of 435 Hz, which had also been the Austrian government’s 1885 recommendation. Johann Heinrich Scheibler recommended A440 as a standard in 1834 after inventing the “tonometer” to measure pitch, and it was approved by the German Natural History Society the same year.
The American music industry reached an informal standard of 440 Hz in 1926, and some began using it in instrument manufacturing.
In 1936 the American Standards Association recommended that the A above middle C be tuned to 440 Hz. This standard was taken up by the International Organization for Standardization in 1955 (reaffirmed by them in 1975) as ISO 16.