At my work, where a baby grand piano resides, the humidity is high, the temperature fluctuates wildly and there are students all over the place like a plague.
Though the piano itself came pretty well in tune, there was one particular key that was sticking for some reason, so we got a repair person in. This person, under some mass miscommunication, ended up tuning the piano, and not fixing the key.
The piano became less in tune than before.
Previously, the notes were all sharp - higher than they should be - but collectively and equally so. This is fine.
The girl who came in used a tuning fork (an outdated practice made obsolete by the humble guitar tuner/smartphone) to tune them down slightly - but still not enough - inconsistently.
I tried to correct parts of the piano which were most obviously out of whack but quickly realized she wasn't that great and, well, lacked an electronic tuner.
Instead, I ordered my own piano tuning kit and figured why not use my uncomfortably empty work schedule to learn a new related and useful skill? I hear it pays well, too.
Annoyingly curious cats aside, here is the kit in all its glory:
I don't really know what most of it is for, but it seems for basic tuning you need only a couple of things: The mutes that stop other strings resonating, the hammer wrench thing that turns the tuning pegs and maybe the red felt strip. I'll demonstrate in this boring video:
I would say you don't really need to be musical in order to accomplish this, but you do need to have trained ears that can hear the oscillations of the waves and how they interact with each other, which I learnt naturally through musicianship, particularly years of tuning a guitar and studio production. For me, I feel I didn't really learn anything other than the confidence to do it. The tuning itself is very similar to other instruments, right down to the tuning hammer thing being basically a drum key.
Since this video, I found the time to finish the piano up, all 88 keys, using an app on my phone as a tuner, and the piano is finally at concert pitch, equally tuned to the best that I believe is possible.
Humidity and other effects are inevitably going to change this over time, but now we don't need to hire a professional to do it over and over, I can just do it for free and improve myself a little more each time!