So when the whole piano is tuned, its nearly impossible to accurately tune the whole instrument to 440, so they adjust the pitch to be a little sharp in one pass to get the piano roughly in tune. When the second person stands on the wire at 441, your pitch drops to ~440. When the tuner tightens the bass notes to 441, that represents the first person standing on the wire. So now we can envision the tight rope is the bridge of the piano, and when there is no one on it the pitch is 435. Their force (in this case, applied by weight and gravity) will add more tension to the wire, and it makes your body rise up a little. Now imagine your friend stands on the wire close to the other tree. You stand on the rope close to one tree and the rest of the wire tightens because of your force bearing down. Imagine a tight rope stretched between two trees with a little slack in the wire. When the piano is very flat, and the technician turns the tuning pin to adjusts the pitch, this new tension creates a new force of bearing across the bridge. But the bridge can be anywhere from three feet long and seven feet long.

The tension of the strings is applied directly onto the bridge in order to create the sound you hear when you play a note.

Your piano has over 230 strings stretching from the pin block down to a bridge and anchored onto the cast iron plate. So why can't it be tuned accurately on the first tuning?

That first tuning is the pitch correction. If it's not close, than it takes two tunings to get it right. What is a pitch correction? In short, when a piano tuner comes over for an appointment, he or she can fine tune the piano in one tuning when the piano is close to pitch.

What is pitch correction