In the orchestra, all instrument begin tuning to the oboe which is playing A=440. The pitch is (relatively) fixed from this instrument; all other strings, brass, reeds & percussion are movable to tune to this.
I worked in a recording studio where they had 4 pianos, tuned once a month. The tuner traveled with 3 forks (A,C,high A), started in the middle and then worked his way out.
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