Above, on the floor, a marker plate is placed in the floor cavity. The idea is to align this marker plate - its tube - in the line of sight, and then pour a suitable mortar all around in that position. In order to carry out the alignment and casting in a controlled step-by-step manner, as well as to ensure a high level of accuracy for the lead, a cross guide is used. This cross guide is bolted to the floor - that is what the two screw sleeves and the flat piece of floor are for. A plumb target fits in the cross guide and in the underlying marking plate which, thanks to guide axes, can be moved in two mutually perpendicular directions and can be blocked in each direction.

If a plumb laser is used for plumbing instead of an optical instrument, the procedure to be followed is broadly the same as described above. In this case, the plumb target consists of a pattern of concentric circles to better centre it in the laser beam. And the target has an adjustable filter to protect the observer's eyes, as the observer is looking straight into the laser beam. There is also an electronic plumb aiming mark - the laser detector - which, based on the position of the laser beam on photocells, simply indicates via illuminated arrows in which direction to look for the centre