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 The initial standard was A = , but this was superseded by A = 440 Hz, possibly because 439 Hz was difficult to reproduce in a laboratory since 439 is a prime number.