Router Plane

A router plane rides flat on the surrounding wood surface on a wide sole, with a small L-shaped cutter projecting down through the sole by a set, adjustable amount. Pushed or pulled across a recess -- a dado, a hinge mortise, the bottom of a tenon's shoulder -- it trims the floor to one exact, dead-flat depth no matter how uneven the initial rough cut was, since the sole's own reference surface controls the depth, not the user's hand.

It fills essentially the same role a hand-held power router does for cleaning up recess floors, minus the speed -- valued specifically where fine control at low speed matters more than how fast the material comes out, like fitting a hinge mortise to a hairline-precise depth.