The approx. 20 tonnes of string tension on a piano is mainly borne by the wooden soundpost via the tuning pegs. Apart from a metal innovation by Crasselt&Rähse at the time, all pianos still have soundposts made of beech wood or multi-layered wood. Even if a metal cover or the cast frame can be seen from the front, the tuning pins are located in the wooden soundpost behind it.
Apart from very rare material weaknesses, a soundpost will be damaged if the humidity is permanently too low and the wood dries out too much. Once a soundpost has cracked, it will not close again stably if the humidity is raised again. Loose hold of the tuning pins or cracks make a stable tuning position or even tuning impossible. In principle, it is always possible to replace the soundpost - but this involves a major investment.
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