We all know that over time the pitch of a piano will drop. If a piano hasn't been tuned in a few years (or longer), it may require a pitch raise. This basically is a rough tuning to bring the piano up to pitch before a final fine tuning. A single piano strings exerts about 160 pounds of pressure on the frame of the piano. With over 200 strings in a piano, this equates to over 16 tons of pressure (even more in larger grands). As time goes by, the strings stretch out, the tension drops and the piano goes flat. As the strings are pulled back up to pitch, the accumulative effect on the entire piano compresses the soundboard and makes the overall pitch drop back down. This is why a rough pass is needed. Without this rough pass, the notes already tuned will drift flat, ruining the tuning.
The longer a piano goes without a tuning, the more it drops in pitch. Pianos that haven't been tuned in many years may need more than one rough pass before they can be properly tuned. Since raising the pitch this much adds so much tension to the soundboard, these pianos usually require a follow-up tuning within a few months and several more tunings before they are truly stable.
