The approx. 20t of string tension of a piano weigh over the tuning pegs, especially on the wooden soundpost. With the exception of a metal innovation by Crasselt&Rähse at that time, all pianos to this day have soundposts made of beech wood or multilayer wood. Even if a metal cover or the cast frame can be seen from the front, the tuning pegs are still located in the wooden soundpost behind it.  

Apart from very rare material weaknesses, a soundpost is damaged if the humidity is permanently too low and the wood dries out too much. A soundpost that has been broken will not close stably when the humidity is raised again. A loose grip of the tuning pegs or cracks make a stable tuning posture or even the tuning impossible. In principle, it is always possible to replace the soundpost - but this involves a larger investment. 

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