What is a Protractor?

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  • Written By: Damir Wallener
  • Edited By: L. S. Wynn
  • Last Modified Date: 13 August 2019
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A protractor is a device for measuring the angle between two intersecting lines. The angle is measured in degrees, and a circle is defined as having 360 degrees of identical size. The earliest known protractors were designed in the 17th century by sailors needing to plot courses on nautical charts.

A basic protractor consists of a half-circular piece of material with degree markings notched onto the outside edge of the arc, or curved section. The straight edge is placed along one of the lines to be measured, and the number of degrees is determined by where the second line crosses the arc.

Because it is a half-circle, the basic protractor only measures angles up to 180 degrees. This is adequate in normal use, as the relationship between two lines can always be expressed as an angle of 180 degrees or less. If the original placement of the protractor produces an angle greater than 180 degrees, simply flipping it over will provide a reading of less than 180 degrees.

This is due to the symmetrical nature of the circle; starting from one line, the angle to a second intersecting line can be measured by moving either clockwise or counter clockwise. One direction will measure an angle greater than 180 degrees, and the other will measure an angle less than 180 degrees. They cannot both measure more than 180, as they must add up to 360, the number of degrees in a full circle.


An alternate type of protractor is made from two straight edges attached to each other at one end, with a graduated arc between them. The arc is attached to one arm, and the other arm slides freely over it. An arm is placed along one line, and the other arm is swung outwards until it is parallel to the other line. The degrees are then read off the arc. Astronomers often use a third type of protractor, one formed from a fully circular piece of material. This allows them to directly measure all angles between 0 and 360.

Protractors are generally used for measuring existing lines, but they can also be constructed to help draw them. By attaching a swiveling arm to the straight edge of the semicircular disk described above, lines with arbitrary angles can be drawn. Such devices are most commonly used in drafting of technical drawings such as architectural plans.


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Post 3

Yes, there is such a thing as a "professional" tractor versus an "amateur" tractor. Most residential customers who live in the country don't want to spend the money for a regular, heavy duty piece of farm machinery, so they opt for a smaller, lightweight tractor to plow their garden patches.

Similarly, if you go to any hardware store, you will find a wide range of power tools. For example, a professional contractor will probably spend several hundred dollars on a quality power tile saw, whereas the DIY homeowner will be more likely purchase a cheap saw for fifty bucks.

The cheap saw won't stand the test of time, but it will suffice for a few tile jobs around the home.

Finally, why is a protractor called a protractor? Because that is the name given to a device used to measure angles. There are many types of protractors, depending on what you plan to do.

A drafter uses a flat protractor for drawing whereas a carpenter uses one with two edges and a dial or digital angle indicator to measure corners.

Post 2

Dude, True! I was thinking this too.

Post 1

Why is a protractor called a protractor? Why not a device to measure freaking angles? We could call it an Angle measuring thingy. Or AMT for short in case you need to text it to someone. Seriously, is there such a thing as a professional tractor and wouldn't it mean there is an amateur tractor? Please help. I am losing sleep over this and my dog keeps looking at me funny!

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