nd in most cases
you will find the C which is two octaves below middle C to be higher
(relatively) than any other C in the piano. If so, take it as a basis
and tune by perfect octaves up to 3C.

The supposition is, that all strings in an instrument gradually grow
flatter; and in a well-balanced instrument they should do so; but the
fact is, that in certain cases some of the strings will grow sharper.
The cause is this: The tension of the strings on one side of a brace
in the metal plate or frame is greater than on the other side; and if
there is any yielding of the structure, the result is that the
overpowered strings are drawn tighter. This condition, however, is
rare in the better grade of pianos. Here is a rule which is safe, and
will prove satisfactory in ninety-nine per cent. of your practice
where no specific pitch is prescribed:

Take the three Cs included in the temperament and the C that is an
octave below 1C, and try each of them with its octave until you
ascertain which is the sharpest with respect to the others; then,
bring the others up to it. You now have your pitch established in the
Cs and can begin on 1C and proceed to set the temperament. Before
applying this rule, it is well to try 3C with tuning pipe or fork to
see if the piano is below international pitch. We would not advise
tuning any modern piano below international pitch. Aim to keep within
the bounds of the two prescribed pitches; never higher than concert,
nor lower than international. If, however, you should be called on to
tune an old instrument that has become extremely low, with very rusty
strings, and perhaps with some of them broken, that by all appearances
will not stand even international pitch, you may be compelled to leave
it somewhat below.

~The Continuous Mute.~--Do not try to set a temperament without a
continuous mute. Its purpose is to mute all outside (1st and 3d[C])
strings of all the trios included in the temperament so that none but
the middle strings sound when struck by the hammers. The advantage of
this can be seen at once. The tuner tunes only the middle strings in
setting the temperament and thereby avoids the confusion of hearing
more than two strings at once. The continuous mute is then removed and
the outside strings tuned to the middle. Without the continuous mute,
he would be obliged to tune all three of the strings of the unison
before he could tune another interval by it, and it would not be so
safe to tune