Plumb lines are mentioned as far back as ancient Egypt, where workers used them to level stones for pyramids and temples. The Bible mentions plumb lines as a tool used by the Hebrews in the books of Amos and II Kings.
Roman, Greek, Persian and Chinese architects and medieval castle builders all found the plumb line an essential tool. Arab surveyors during the time of the Caliphate invented a wooden board with a plumb line and two hooks and an equilateral triangle with a plumb line and two hooks to use for reading inclines and leveling structures.
Even until the 20th century, virtually every builder of tall structures depended on plumb bobs and plumb lines to get his constructions straight.
Plumb lines provide a fixed vertical reference point for tall structures. Engineers used to tie the upper end of the plumb line to a movable scaffold atop the structure so the plumb bob dangled directly over a reference datum mark on the floor at the base of the building.
As the building grew taller, the scaffold rose, and the plumb line was used as a fixed central measuring point from which workers laid out the upper sections of the structure. To this day you can find brass plates fastened to the floors of older buildings with etched datum marks for the plumb lines.
Builders still use plumb lines as guides to building exactly vertical structures. In recent years, however, laser levels and electronic measuring tools have begun to replace the traditional plumb line.
Level and Inclinometer
A plumb line by itself can only determine a vertical line, but if mounted on an instrument with a horizontal scale against which the plumb bob can swing back and forth, the plumb line can be used to determine the angle of a plane surface. The ancient Egyptians leveled stone blocks with an A-frame device that had a plumb line and plummet hung from the apex of the "A" The mason could set the device atop a stone block and use it to tell if it was exactly level or not, based on where the plumb line touched the cross-member of the "A".
Artists and designers use plumb lines to find the vertical axis through the center of gravity of subjects with irregular shapes. This line becomes a useful point of reference to help the artist line up anatomical elements of the drawing or design and as an aid in visualizing how a subject will move with reference to its center of balance.
The plumb line has long been used as a symbol of uprightness, strength and stability. The symbol appears in the Old Testament, and in Christian and pagan contexts.
Freemasons use the plumb line as one of the symbolic tools presented to leaders as a symbol of rectitude, honesty and direct action. The symbol appears in other systems of faith, governments or associations of individuals to represent ideas such as stability, truthfulness, righteousness, strength, uprightness and moral character, and also as a path to immortality.