A
Tuning
Every string is brought back into relation with the others and with standard pitch, by ear and against a reference, one unison at a time. An instrument that has drifted a long way flat cannot be pulled up and left stable in a single pass: the first pass brings it roughly to pitch, the second settles it, and you are told before any of that starts whether yours is in that position.
Where an old instrument will not safely hold standard pitch, it is tuned to the pitch it does hold, and the reason is written down rather than argued about.