Difference Between a Ruler and a Straightedge

Ruler and straightedge both belong to a category of mechanical instruments used to measure lengths, spaces, and distances.

They are also used for various other purposes in drawing, constructing, and verifying physical factors of other objects.

Ruler vs Straightedge

The main difference between ruler and straightedge is that because of the difference in shape and usage, a ruler can perform the functions of a straightedge but the converse does not hold true. A straightedge cannot be used in the same way as a ruler for some specific functions.  A ruler is a subtype of the straightedge, distinguished by markings along its length.

Ruler vs Straightedge

A ruler is a technical device that is used in mathematics or geometric drawings to draw straight lines with absolute accuracy.

Rulers may be made of various materials like steel, plastic, or wood, and be of different lengths. But they all have standard measuring techniques inscribed on to them, usually centimeters, millimeters, and inches.

A straightedge is a tool often used for more large scale industrial and mechanical projects. It is also used for checking the straightness of other surfaces and already drawn lines.

It is a narrow instrument with no gradations drawn on to it, and thus cannot be used to measure lengths and spaces like a ruler.

Comparison Table Between Ruler and Straightedge

Parameter of ComparisonRulerStraightedge
Practical usageTo draw lines and measure accurate lengths using the gradations marked into it.To draw lines or measure the straightness of other lines and surfaces.
Technical usageMostly used by students and other professionals for geometrical diagrams or graphs.Mostly used for industrial purposes, such as the automotive industry or machining services.
Shape of instrumentA ruler has a proper width between two clearly defined parallel lines. It can be rectangular or triangular.A straightedge is simply a narrow stick-like instrument that does not have a specified width based on parallel lines.
Size of instrumentRulers can be of different lengths, as longer rulers are used to measure longer lengths.Straightedges usually come in industrial standard sizes and as such are heavier than rulers.
Material and AvailabilityRulers are generally made of steel, plastic, or wood and are very commonly available in stationery stores.Straightedges can be made of a variety of materials such as aluminum or cast iron. The ones manufactured for industrial use are not easily accessible.

What is a Ruler?

A ruler is an instrument of scale used for measurement and accurate drawings in the fields of mathematics, geometry, engineering, and construction.

Rulers are always inscribed with accurate gradations and often come in different shapes to facilitate its purpose. Sometimes line rods and measuring tapes are casually referred to as rulers but that is not a correct usage of the term. 

Rulers can be used for regular practical purposes at home as well, such as for folding paper into equal halves or for cutting through sealed envelopes.

Its importance for geometric deductions is undeniable since it has been used to prove various theorems instrumentally on paper.

It has been used in several ruler and compass constructions, most notably in the division of angles in two or more parts.

Today rulers are used in projects by all students and are an integral part of every geometric box set found in the markets.

The earliest known example of a ruler is perhaps a bar with gradations marked on it found at the excavation site of a Sumerian city.

The concept of a ruler has been borrowed for usage both practically (architect’s scale and engineer’s scale) and in philosophical discussions about accuracy in signification.

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What is a Straightedge?

This instrument is necessarily sharp edged, and used for precision in determining the flatness of surfaces and straightness of lines.

It is thus of utmost importance in industries, factories, and for construction where even the leveling of machine surfaces needs to be confirmed.

One of the most important uses of this tool in experiments is in the straightedge and compass constructions.

The straightedge has other physical uses in construction as it is often combined with other machinery such as saws while cutting through dense material.

In the specialized study of metrology, the straightedge can be divided into the four following categories – tool maker’s straightedge, angle straightedge, wide edged straightedge, and the box straightedge.

Using this tool is very simple because all one has to do is place it above a surface and view the arrangement against the light, or place it along the length of any line that is to be checked.

By following the length of the straightedge it can be easily understood if the surface is in question is level enough or the diagram drawn is in line with the straightedge.

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Main Differences Between a Ruler and a Straightedge

  1. The primary distinction between the two is that a ruler is a kind of straightedge in which there are gradations marked for enabling accurate measurements.
  2. Structurally the two instruments are quite dissimilar, since they also have different uses. The ruler has two parallel lines and is usually rectangular in shape, whereas the straightedge is mainly sharp, pointed, and has a thin body.
  3. The straightedge can only be used to find out if a particular surface is flat, or if a line is straight. This is very useful in industrial uses because several other machines require this feature to precisely work on the raw materials. The ruler on the other hand is used for accuracy in length measurements and drawing straight lines.
  4. Students generally prefer a ruler over s straightedge as it serves multiple purposes, unless there is a specific experiment which requires the straightedge.
  5. A straightedge is usually always bulkier and heavier than a ruler depending on the material out of which it is made and the use for it is built.
Difference Between a Ruler and a Straightedge


The straightedge and ruler are similar instruments that can usually be found in related areas of work. Since the ruler is handier and can be used for more purposes, it is generally found more commonly.

However to a professional, both tools are indispensable for determining precision in measurements.

Although the words are often used interchangeably, as each of the two tools can sometimes be used in place of the other for certain functions, they are distinct in the ways elucidated above.

However, since a ruler is a more specialized form of the straightedge, it is usually more useful to the layman.


  1. https://digitalworks.union.edu/theses/1563/
  2. https://core.ac.uk/download/pdf/322471216.pd

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