Larry Schuette asks about yellowing of ivory piano keys.  I can tell
him from personal experience that keeping the keyboard cover closed
does, indeed, cause yellowing and leaving it open allows them to revert
to white.

When I was a child we were given a German grand piano by my grandfather.
As this had been rarely played for many years and the keyboard cover
had been kept closed, the keys were all badly yellowed.  We installed
it near a large window and our piano tuner told us to leave the cover
open, so that is what we did.  We all learnt to play on this piano and
always left the cover open.  After a few years the keys had reverted to
a brilliant white.

Another advantage of leaving the cover open is that it encourages
people, especially children, to play.  I never close the keyboard cover
on any of my pianos and any visiting children are never discouraged
from having a go, in spite of their parents telling them not to.

Nicholas Simons, GB