As classical pianist there is no doubt you are interested in how your piano basically works.

Usual string configuration in a piano

First, open your piano and have look inside. (beware not to break something, specially the white little hammers are fragile!)

You’ll see that the strings are grouped by 3 — well, that is, for most of them.

Actually if you start to the left, the strings are very big and are not grouped at all. Each string is one note. Just push on the right pedal with your foot and gently pluck the big strings one by one : one string, one note.

Go a dozen of centimeters (more or less 5 inches) to the right , about ten strings, and you’ll see that the strings start to be grouped by pairs! Keep your foot on the right pedal and pluck them, you’ll hear they sound the same pitch.

Shift furhter still some more to the right, about 20 centimeters or 8 inches, and there they are, all grouped by 3. Keep playing with the pedal, pluck and play the notes on the keyboard. The hammer strikes 3 strings in one time. If you go back down and keep playing the keyoard, you’ll see that the hammer always strikes a group of strings, or a single string in the lowest part.

The pitch of the piano strings

Strings vary in length, mass and tension. A change to any of these factors changes the pitch.  That’s why the lowest are so long, thick and loose, compared to the short, thin and rigid ones of the highest notes.
To tune a piano, the piano tuner winds the tune a bit more on some kind of screw (the tuning pins) if he wants to raise the pitch. If he wants to lower the pitch, he turns the screw ant-clock-wise in order to decrease the tension on the string.