It would be interesting to know why key 'A' on any International standard keyboard instrument is of frequency 440 Hz.

A European Musician and Composer Paul Winter had visited the Egyptian Pyramids. In the 'Kings Chamber', he found that the sound made by hammering the sacred stone called as 'Sacrophagus' was at 440 Hz.

The Pyramids were built in around 3500 BC. 

This is the reason that the first note 'A' on the Piano was tuned to 440 Hz. Egyptians were very advanced in Mathematics, Music, and Astronomy, and this frequency may have some spiritual application. If 'A' is at 440 Hz, 'C' comes at 261.6255528 Hz.

Earlier to Paul Winter, various ancient Pianos in old churches in Europe show the key 'C' still playing at 256, 262, 264, 278.4375, and 280 Hz. These differences were unanimously removed in an International Conference held in Europe in 1939 where 'A' was fixed at 440 Hz.