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A half-circle protractor marked in degrees (180°). A protractor is a measuring instrument, typically made of transparent plastic, for measuring angles. Some protractors are simple half-discs or full circles. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle.
For example, the degree is defined such that one turn is 360 degrees. Using metric prefixes, the turn can be divided in 100 centiturns or 1000 milliturns, with each milliturn corresponding to an angle of 0.36°, which can also be written as 21′ 36″. [16] [17] A protractor divided in centiturns is normally called a "percentage protractor".
In trigonometry, the gradian – also known as the gon (from Ancient Greek γωνία (gōnía) 'angle'), grad, or grade [1] – is a unit of measurement of an angle, defined as one-hundredth of the right angle; in other words, 100 gradians is equal to 90 degrees.
T bevel with protractor and dividers In use. A sliding T bevel, also known as a bevel gauge or false square [1] is an adjustable gauge for setting and transferring angles. . Different from the square, which is fixed and can only set a 90° angle, the sliding T bevel can set any angle and transfer it on anoth
The protractor head has a flat reference edge which is attached to an adjustable 180° protractor or (sometimes called a turret) with a graduated scale in both directions for reading both the angle or the complement angle. [1] The protractor head sometimes includes a small spirit level. [4] [2]
Similar to the Geodreieck, a number of other protractor triangle types exist for navigation purposes. Various designs are named navigation (protractor) triangle, nautical navigational triangle, nautical set square, Portland (navigational) triangle or Portland protractor triangle, Kent-type triangle, Inoue-type A/B nautical triangle or plotting triangle, course triangle, yachtsmen triangle, and ...
For example, an angle of 30 degrees is already a reference angle, and an angle of 150 degrees also has a reference angle of 30 degrees (180° − 150°). Angles of 210° and 510° correspond to a reference angle of 30 degrees as well (210° mod 180° = 30°, 510° mod 180° = 150° whose supplementary angle is 30°).
The sum of the angles equals 90 degrees (22.5° +67.5°= 90°). It is also obvious from a visual check when using a protractor that where the instruments displays 22.5° is actually 67.5° on the protractor. Many newer slide miters and miter boxes display both angles. Some of the new calculators have a 0° and a 90° references.