Historic tuning references can be found in antique tuning forks (corrosion effects?) and organ pipes. A440 is a modern beast. A415 is used by many Baroque playe

Historic tuning references can be found in antique tuning forks (corrosion effects?) and organ pipes. A440 is a modern beast. A415 is used by many Baroque players. A392 may have been in use by the clavecinistes in the 18th century. Then there's a multitude of ways to split the octave. As far as ET is concerned, a change in key is simply shifting the register by a number of semitones. In non ET tunings, the interval configurations change with the keys. With some mean tone tunings, you retune some strings to accommodate different keys.