Levelling along a plumb line - one of the features of the MOUS System - assumes a free sight path to higher floors. This free sight path requires recesses in the concrete floor. Within the MOUS System, a floor recess has a special shape, which is obtained by using a special recess template in combination with a piece of PVC pipe; the pipe, which is cut to a length determined by the floor thickness, acts as lost formwork material (figure). The sparing jig is centred on the underlying floor formwork above a setting board nailed to it or directly above a drilled hole in the formwork board. For wide slab floors, a plate of aerated concrete is usually cast in with the formwork, into which a hole is cut on site using a hole saw, after which the supporting template can be centred above it. The recess template may be inserted into the floor with an inaccuracy of +/- 20 mm, leaving a sufficiently clear sight path in the floor cavity. In addition to guaranteeing the 'correct' position of the inspection opening, the recess jig allows a shallow recess to be made in the top of the floor to be poured and a flat section of floor to be created with two screw sleeves in it. Also in certain types of precast concrete floor elements - no, not in hollow-core slabs, for which there is a special solution - the typical mousing recess is applied in the concrete plant in a similar way.