How does a piano tuner tune a piano?

Question

How does a piano tuner tune a piano?

Answer 1

Some people have what is called "perfect pitch".  It causes them to be able to recognize if a note is sharp or flat.  After they tune the middle C key, they can tune all of the others from that.

Answer 2

Karen Burns, Working Girl » a piano tuner bears it all.

Piano tuning isn’t really a thing you can learn from a website.  There’s a lot to it.  You have to learn how to use a tuning lever on different kinds of tuning pins—very loose, jumpy, or whatever—and you need to develop your hearing so that you can hear the various aural tests to confirm your tunings are good.  It’s much more complicated to tune than people think.

A person with perfect pitch isn’t listening the same way a piano tuner does and doesn’t need to be as accurate as a piano tuner has to be.