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One of the problems with leaded windows, particularly those made up from
small panes is that the woodwork can't use the stiffness of the glass to
help it resist drooping on the diagonals.

Glass is never an exact fit in the opening, so by this reckoning all
doors are doomed to droop a few mm.

And indeed they often do, particularly when they're old and the tenons
have shrunk.

--
Skipweasel
Pay no attention to that man behind the curtain.