Piano Frame:

In order for the strings to be tuned to high tension and to hold their tuning, a metal frame was introduced to the piano. Instruments in existence before the piano, such as the harpsichord, didn't have an iron frame. Pianos were originally all wooden and there are still many pianos in the UK by firms such as Broadwood with little or no iron frame. At first the part where the strings were attached was made of iron, then gradually more iron was introduced until the full iron frame was first introduced by Steinway in 1877.

The metal frame or plate is generally made of cast iron.