Tuning a piano involves the setting of each of the 220+ strings so that they sound in tune with each other. Each string is pulled to a tension on average of 90 kg. this puts the total tension in a piano at around 20 ton. It is up to the cast iron frame to keep the instrument from collapsing under the immense pressure.
Pianos that haven’t been tuned for a few years and are more than a few cents flat may need a pitch raise. This involves roughly tuning the instrument to bring it to concert pitch then doing a normal tuning to stabilise the piano.
Most pianos built from 1910′s-20′s onward should be designed to be tuned at concert pitch (A above middle C = 440 hertz). Older pianos may not be able to cope if they are designed to be tuned at a lower pitch. If the piano is tuned too sharp strings can snap, the frame can crack and all this can put the technician and you in danger.
Therefore it is important for your piano to be thoroughly examined before tuning commences if it is an older instrument.