Edited sentence: A theodolite is mounted on its tripod head by means of a forced centering plate or tribrach containing four thumbscrews, or in some modern theodolites, three, for rapid levelling.Before use, a theodolite must be placed precisely and vertically over the point to be measured—centering— and its vertical axis aligned with local gravity — leveling.The former is done using a plumb bob, spirit level, or optical or laser plummet.Their existence is taken into account in the choice of measurement procedure in order to eliminate their effect on the measurement results