In MMD 981107 Reyah Carlson asked what and where the Themodist is:

The Themodist (Aeolian's trademark), as well as other maker's similar
systems like Solodant, is a device which causes most of the notes in a
piece to play at a constant, low volume.  Certain notes, usually the
main melody line as well as some ornamentation, are coded on the roll
to be played louder, their volume controlled by the pianolist's
pumping.

The playing vacuum supply goes through a regulator, similar to a roll-
speed governor; and then is piped to the two halves of the playing
'stack' (the pneumatic action), divided into bass and treble sections.
Thus the entire piano will usually play at the level set by this
regulator, regardless of how hard one pumps.

When a note or chord is to be accented, a pair of small perforations
at the edge of the roll (one pair for bass, and one for treble) cause
a bypass valve to open and 'short-circuit' the regulator for that side
of the action.  When this occurs, the full vacuum created by the
pianolist's pumping plays those notes, as loudly as one wants.

Manual bypass valves, operated by levers on the key-slip, are located in
parallel to these automatic valves, so that the user can increase the
background level.  Also, these valves can be fully opened, taking the
Themodist out of the circuit, so that the entire roll will play under
the control of the user's pumping.  This feature is needed when playing
the majority of rolls which are not 'snakebite coded'.  Sometimes push-
buttons or short-stroke levers are provided which open the 'snakebite'
ports, allowing the Pianolist to add accents manually.

Physically, there are three components of the system:

The special tracker bar, with two horizontally elongated holes; one
to the left of the first playing note, and the other to the right of
the 88th playing note.

The primary valve box, a cube about 3 inches wide, usually located on
the upper deck to the left of the spool box.  Since the Themodist
bypass valves have large pouches, and must work very quickly in order
to open and close for only one accented note or chord, they are actuate
through 'relay' primary valves, (very precisely made, with adjustable
valve gaps) located in this box.

The regulator and bypass valve assembly, usually hung under the keybed
on the left side of the piano.  This is a rectangular box, about 8
inches wide, with a spring-loaded bellows forming its bottom.  A large
hose goes from the pump assembly to this box, and two smaller hoses go
from this box to the player; one up and to the left for the bass
supply, and the other across the underside of the keybed to the right
end for the treble side.  If this assembly is located elsewhere in your
piano, it can be located by tracing the rods from the manual control
levers, which may be labeled 'bass-treble / loud-soft' or some similar
language.

If you can't find these items, and large hoses go directly from the
pump assembly to the action.  There is a chance that someone in the
past ripped out the Themodist, which interferes with the playing of
modern rolls if you don't know how to use it properly.  Unfortunately,
this has happened.

Richard Vance