John Harrold wrote in 110221 MMDigest:

"... there seems to be confusion about A=435 Hz and A=440 Hz tuning.
The 1937 international symposium on tuning adopted as standard 435 Hz
at 15 degrees centigrade.  This had been used by the majority of
builders (but not all) for some years.  This is very close to 440 Hz
at 20 degrees centigrade, so you can see that there is not a great deal
of difference given today's warmer buildings."

If we are talking about pianos made in the USA, I think it is safe to
say they were built to handle A=440 tension since the late 1800's, and
certainly since WW1.

Also note that piano pitch drops as temperature rises (at least at
first), which is the opposite of organ pipes.  Eventually the plate
expands about as much as the strings do, so the pitch is often a wash
at equilibrium, with various sections of the piano responding
differently.

The A=435 standard at 15 degrees vs. 440 Hz at 20 degrees may matter to
pipe organ builders, and perhaps to orchestras, but not to pianos.
Manufacturers of current day pitched percussion instruments tune at or
above 440 Hz, often 442 Hz.  Pianos today (and many brands going back
100 or more years) are all designed for A=440.  I'm confused about who
"the majority of builders" are that John mentions.

Regarding the dangers of lowering pitch, only fine, concert quality
grands would suffer if pulled up from 435 to 440, then dropped back to
435 (not that this would be a likely scenario). The string bends at the
bearing point near the tuning pins would slide back into the speaking
length and potentially introduce false beats and poor hammer/string
mating.  This is a big deal for concert venues which try to appease
visiting orchestras that tune to A=442 or higher, when the pianos are
normally maintained at 440 Hz.  _But,_ on lesser instruments, the
strings are not that pure to begin with, and routinely move throughout
the seasons, so I have a hard time believing any Seeburg nickelodeon
would suffer a noticeable change in tone after dropping the pitch, if
so desired.

Keep in mind that normal seasonal humidity changes can produce pitch
changes greater than the difference between 435 Hz and 440 Hz (19.8
cents), so those strings are moving "weather" we like it or not (pun
intended).  If the Seeburg is tuned in February at A=435, it could
easily be A=443 in July.

I personally would tune the piano to the xylophone, since the piano's
pitch is much easier to change, and easy to change back if we don't
like it.

Greg Graham, RPT