Hello Laura,  Thank you for an excellent question.  This is a basic
item that is often misunderstood.

In a _typical_ (and not all are typical) double comb box such as a
Polyphon, two musical combs face each other from opposite sides of the
row of star wheels.  The star wheels are lined up on a single spindle
and held in place by the gantry.  Each star wheel is assigned to
a single tooth on one of the combs and rotates a partial turn when
engaged by a disc projection, plucking its assigned tooth.  As
expected, each star wheel rotates in the same direction when engaged
by a disc projection.  All star wheels are identical in shape.

Each of the two combs is set at a different height above the bedplate
so we speak of the lower comb and the upper comb.   The lower comb is
plucked downwards and the upper is plucked upwards.

For example, on an upright Polyphon, the upper comb is to the right
of the star wheels and the lower to the left.  If you look along the
line of star wheels from the edge of the disc to the centre you will
see that, when in use, a star wheel will rotate in a counter-clockwise
direction.  Thus, a rotating star wheel will engage the upper comb
in such a way as to lift a tooth with an upward stroke but another
independent star wheel rotating in the same direction but engaging the
lower comb will depress a tooth with a downward stroke.  The angles of
attack are set so that the musical effect is similar from plucking
a tooth on either comb.

If you look closely at the star wheel gantry you will see that each
star wheel is assigned to one tooth only, alternating upper and lower
comb as you progress along the line of star wheels.  This arrangement
allows twice as many comb teeth to be accommodated  for a disc of a
given diameter than would otherwise be the case.  In general, disc
musical box teeth are designed to play more loudly (there are
exceptions) than cylinder box teeth and so have to be more robust,
meaning that combs cannot be made with teeth  as close together as
for cylinder boxes.  There is also a physical limit as to how close
together star wheels (and disc projections) can be set without
interfering with each other.

A common variation from the above is the Symphonion double-comb box
which has an arrangement with two similar ("upper") combs and gantries
are used, but in line on diametrically opposite sides of the centre
post.  Discs are arranged to accommodate this 180 degree differential.

By the way, a double-comb box does not automatically imply a sublime
harmony arrangement, a common misconception.  There are some boxes in
which each star wheel is assigned to pluck two teeth simultaneously,
one in each of two combs (e.g. upper and lower).  That is a sublime
harmony setup.

I hope this answers your question.

Best regards,

Jim Heyworth,
Sechelt, B.C., Canada