Being able to draw angles without the aid of a protractor is an important architectural and mathematical skill. Many angles can be drawn with a compass, also known as a pair of compasses. A compass is a drawing tool with two pivoting legs, one ending in a spike and the other ending in a pencil point. The ratio of the circumference of a circle to its diameter is very close to 3:1, and because 120 degrees is a third of a full circle, a pair of compasses can be used to draw a 120degree angle.
Things You'll Need
 Drawing compass
 Ruler
 Sharp pencil

Mark a dot on a sheet of paper. Draw a circle using a compass and a sharp pencil, using the dot as the center point. Take care not to alter the compass setting when the circle is complete.

Place a ruler over the circle with the edge passing through the center of the circle and any point on the perimeter of the circle. Make a clear mark where the ruler crosses the circumference.

Ensure that the radius of the compass has not changed, and then place the pivot point on the mark where the straight line crossed the circle perimeter. Draw a new circle.

Draw straight lines from the center of the first circle to the two points where the second circle crosses the perimeter of the first circle. This creates the points of two equilateral triangles, with internal angles of 60 degrees. The angle between these two lines is 120 degrees.
Tips & Warnings
 Many different angles can be drawn using a compass and a ruler, including those measuring 30, 45, 60, 90 and 180 degrees.
 The radius of the compass must remain unchanged throughout the process. If it changes, reset it to the original value.
References
 "Webster's New World College Dictionary 4th Edition"; Michael Agnes, ed.; 2007
 Arizona Department of Education: Geometry  An ADE Mathematics Lesson
 Minnesota West Community & Technical College: Math 1113 PreCalculus, Circles and Radian Measure, Part I; D. Matthews; December 2006
 Andrews University; A Review of Basic Geometry  Lesson 3 Angles, and More Lines; Keith G. Calkins
 University of Georgia  Mathematics Education; Euclidean Constructions Using Straightedge and Compass; Dawn Leigh Anderson
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