Plumb bob

Until the modern age, plumb bobs were used on most tall structures to provide vertical datum lines for the building measurements.

Many cathedral spires, domes and towers still have brass datum marks inlaid into their floors, which signify the center of the structure above.

An archipendulum, an A-frame level with a plumb line hung from the vertex was also used to find horizontal; these were used in Europe until the mid–19th century.

[further explanation needed] A plumb bob may be in a container of water (when conditions are above freezing temperatures), molasses, very viscous oils or other liquids to dampen any swinging movement,[4] functioning as a shock absorber.

Students of figure drawing will also make use of a plumb line to find the vertical axis through the center of gravity of their subject and lay it down on paper as a point of reference.