This is a fancy name for the seat that has a hole in it to
receive the mast.
mast partner should also be cut to fit, and make sure you leave the center of
the mast hole at 50" from the bow as shown in the rig sketch. If you decide
to make the
rectangular mast from 2x4, make sure the long dimension is athwartship (side to side) as
the mast takes the most load in that direction and is stronger that way. The
mast partner should not be removable but fastened it securely to cleats as a lot of torque is
transmitted to the hull through this piece.