From the Wikipedia:
In 1939, an international conference recommended that the A above middle C be tuned to 440 Hz, now known as
concert pitch. As a technical standard this was taken up by the
International Organization for Standardization in 1955 and reaffirmed by them in 1975 as
ISO 16. The difference between this and the
diapason normal is due to confusion over the temperature at which the French standard should be measured. The initial standard was A =
439 Hz (
info), but this was superseded by A = 440 Hz after complaints that 439 Hz was difficult to reproduce in a laboratory because 439 is a
prime number.
[7] [
edit]Current concert pitches Despite such confusion, A = 440 Hz is the only official standard and is widely used around the world. Many orchestras in the
United Kingdom adhere to this standard as concert pitch.
[8] In the
United States some orchestras use A = 440 Hz, while others, such as
New York Philharmonic and the
Boston Symphony Orchestra, use A = 442 Hz.
[9] The latter is also often used as tuning frequency in Europe,
[2] especially in
Denmark,
France,
Hungary,
Italy,
Norway and
Switzerland.
[10] Nearly all modern symphony orchestras in
Germany and
Austria and many in other countries in
continental Europe (such as
Russia,
Sweden and
Spain) tune to A = 443 Hz.
In practice the orchestras tune to a note given out by the
oboe, and many oboists use an electronic tuning device. When playing with fixed-pitch instruments such as the piano, the orchestra will generally tune to them—a piano will normally have been tuned to the orchestra's normal pitch. Overall, it is thought that the general trend since the middle of the 20th century has been for standard pitch to rise, though it has been rising far more slowly than it has in the past. Some orchestras like the
Berliner Philharmoniker now use a slightly lower pitch (443 Hz) than their highest previous standard (445 Hz).
[11][2] Many modern ensembles which specialize in the performance of
Baroque music have agreed on a standard of A = 415 Hz.
[2] An exact equal-tempered semitone lower than A = 440 would be 440/21/12 = 415.3047 Hz; this is rounded to the nearest integer. In principle this allows for playing along with modern fixed-pitch instruments if their parts are transposed down a semitone. It is, however, common performance practice, especially in the German Baroque idiom, to tune certain works to Chorton, approximately a semitone higher than A-440 (460–470 Hz) (e.g., Pre-Leipzig period cantatas of Bach).
[12]