Hal Davis sent an informative piece on piano shapes, and he described
a weakness in square pianos as "Due to its four sides, it could change
its shape without changing any of the dimensions of the sides.  This
is primarily the fault with the square piano."

How then, do upright pianos avoid this problem ?

Just curious

Harvey Chao

 [ The cast iron plate was added inside the horizontal 'grand' piano,
 [ and the strings were attached to this casting instead of to a wood
 [ frame.  The tuning stability was thereby greatly improved.  Vertical
 [ pianos with cast iron plates/frames soon followed.  -- Robbie