The tuning fork is based on the key of A ( 440mghz ) If I restring my guitar I can retune it without any external assistance this called dperfect pitch.Once the pitch of A had been established Tuning forks wer created to match that pitch from a violin.Once you had one single tuning fork in pitch then all you had to do was continually match it .
If you can listen to a song on the radio and remember the pitch five days later then you understand how it works