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What is the Plumb Line?

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The plumb line, also known as the “ruler of uncertain proportion,” is used in construction. It has two main uses: one to measure distances on building sites; another for ensuring that parts fit correctly when assembled by using its height measurements alongside other instruments such as levels or spirit levels – all three are commonly referred to collectively as “instruments of the establishment.”

The ruler’s length varies depending upon what task it will be put forth during any given project but typically ranges anywhere between 20-30 ft.


Before the modern era, plumb bars were used on most tall structures to provide vertical reference lines for building measurements. A plumb line was installed on a section of scaffolding and centred on the reference mark on the floor. As the building went up, the plumb line also increased, still centred on the reference point. Many spires, domes and towers of cathedrals still have brass marks embedded in the floor that marks the center of the structure above.

Plummet and ruler as an inclinometer

A plummet and ruler by themselves can only determine the vertical coordinate. However, if you set them on a suitable scale, the instrument can be used as an inclinometer to measure angles to the vertical.

The ancient Egyptians used a plumb bob attached to the top outside of an instrument resembling the letter E; the plumb line indicated a vertical line when applied to a wall. An A-shaped level with a plumb line suspended from the top was also used to determine the horizontal; such levels were used in Europe until the mid-19th century. A variation of this tool is a plumb line suspended from the top of an inverted T.

Early skyscrapers used heavy plumb bars suspended from wires in elevator shafts.

The plumb line could be in a water container (if the temperature is above freezing), molasses, very viscous oil or other liquid to dampen any oscillating motion, functioning as a shock absorber.

Determining the center of gravity of an irregular shape

Students involved in figure drawing also use a plumb line to find the vertical axis passing through an object’s center of gravity and mark it off on paper as a point of reference. This can be done using special plumb bars or simply homemade devices made from string and a weighted object (such as a metal washer). This plumb bob is essential for constructing anatomical geometry and visualizing the subject’s center of balance.