Compass - math word definition - Math Open Reference

A compass is a drawing instrument used for drawing circles and arcs. It has two legs, one with a point and the other with a pencil or lead. You can adjust the distance between the point and the pencil and that setting will remain until you change it.

Along with a straightedge, the compass is used extensively in constructions, where geometric operations are carried out by drawing only, with no calculations involved.
See Constructions using a straightedge and compass

This kind of compass has nothing to do with the magnetic compass used find the north direction when you are lost.

Pages referring to 'compass'

Constructing 75, 105, 120, 135, 150 degree angles and more. Euclidean constructions with compass and straight edge (ruler). The table shows angles that can be obtained by combining simpler ones in various ways

How to construct (draw) a regular pentagon inscribed in a circle. The largest pentagon that will fit in the circle, with each vertex touching the circle.

A brief introduction to constructions - creating various geometric objects with only a compass and straightedge or ruler. History and origins

How to copy a line segment with compass and straightedge or ruler. Given a line segment, this shows how to make another segemnt of the same length. A Euclidean construction.

How to divide a line segment into equal parts with compass and straightedge or ruler. We start with a given line segment and divide it into any number of equal parts. In the applet we divide it into five parts but it can be any number. Using a compass and straightedge, we do this without measuring the line. A Euclidean construction

How to find the center of a circle with compass and straightedge or ruler. This method relies on the fact that, for any chord of a circle, the perpendicular bisector of the chord always passes through the center of the circle. By applying this twice to two different chords, the center is established where the two bisectors intersect. A Euclidean construction

This construction shows how to draw the perpendicular bisector of a given line segment with compass and straightedge or ruler. This both bisects the segment (divides it into two equal parts), and is perpendicular to it. The proof shown below shows that it works by creating 4 congruent triangles. A Euclideamn construction.

How to bisect an angle with compass and straightedge or ruler. To bisect an angle means that we divide the angle into two equal (congruent) parts without actually measuring the angle. This Euclidean construction works by creating two congruent triangles. See the proof below for more on this.

On this page we show how to construct (draw) a 90 degree angle with compass and straightedge or ruler. There are various ways to do this, but in this construction we use a property of Thales Theorem. We create a circle where the vertex of the desired right angle is a point on a circle. Thales Theorem says that any diameter of a circle subtends a right angle to any point on the circle. A Euclidean construction.

This page shows how to construct (draw) the circumcircle of a triangle with compass and straightedge or ruler. The circumcircle of a triangle is the circle that passes through all three vertices of the triangle. It's center is called the circumcenter, which is the point where the three perpedicular bisectors of the sides intersect. A Euclidean construction.

How to construct (draw) the centroid of a triangle with compass and straightedge or ruler. The centroid of a triangle is the point where its medians intersect. It is also the center of gravity of the triangle and one of the triangle's points of concurrency. It works by constructing two medians, which intersect at the centroid. A Euclidean construction.

This page shows how to construct (draw) a 45 degree angle with compass and straightedge or ruler. It works by constructing an isosceles right triangle, which has interior angles of 45, 45 and 90 degrees. We use one of those 45 degree angles to get the result we need. See the proof below for more details. A Euclidean construction.

This page shows how to draw a perpendicular at a point on a line with compass and straightedge or ruler. It works by effectively creating two congruent triangles and then drawing a line between their vertices. A Euclidean construction.

This page shows how to cosntruct a perpendicular at the end of a ray with compass and straightedge or ruler. This construction works as a result of Thales Theorem. From this theorem, we know that a diameter of a circle always subtends a right angle to any point on the circle, so by using it in reverse we produce a right angle.

This page shows how to construct a perpendicular to a line through an external point, using only a compass and straightedge or ruler. It works by creating a line segment on the given line, then bisecting it. A Euclidean construction.

This page shows how to construct an equilateral triangle with compass and straightedge or ruler. It begins with a given line segment which is the length of each side of the desired equilateral triangle. It works because the compass width is not changed between drawing each side, guaranteeing they are all congruent (same length). It is similar to the 60 degree angle construction, because the interior angles of an equilateral triangle are all 60 degrees. A Euclidean construction.

This page shows how to draw the two possible tangents to a given circle through an external point with compass and straightedge or ruler. This construction assumes you are already familiar with constructing the perpendicular bisector of a line segment.

How to construct (draw) the incircle of a triangle with compass and straightedge or ruler. The three angle bisectors of any triangle always pass through its incenter. In this construction, we only use two, as this is sufficient to define the point where they intersect. We bisect the two angles and then draw a circle that just touches the triangles's sides. A Euclidean construction.

Median of a triangle construction with compass and straightedge or ruler. A triangle has three medians. They are lines linking each vertex to the midpoint of the opposite side. We first find the midpoint, then draw the median. A Euclidean construction.

How to construct the orthocenter of a triangle with compass and straightedge or ruler. The orthocenter is the point where all three altitudes of the triangle intersect. An altitude is a line which passes through a vertex of the triangle and is perpendicular to the opposite side. A Euclidean construction

This page shows how to construct a line parallel to a given line through a given point with compass and straightedge or ruler. This construction works by creating a rhombus. Since we know that the opposite sides of a rhombus are parallel, then we have created the desired parallel line. This construction is easier than the traditional angle method since it is done with just a single compass setting. A Euclidean construction.

How to draw an isosceles triangle given the base and altitude with compass and straightedge or ruler. The base is the unequal side of the triangle and the altitude is the perpendicular height from the base to the apex. It works by first copying the base segment, then constructing its perpendicular bisector. The apex is then marked up from the base. A Euclidean construction.

This page shows how to construct (draw) a 30 degree angle with compass and straightedge or ruler. It works by first creating a rhombus and then a diagonal of that rhombus. Using the properties of a rhombus it can be shown that the angle created has a measure of 30 degrees. See the proof below for more on this. A Euclidean construction.

This page shows to construct (draw) a 30 60 90 degree triangle with compass and straightedge or ruler. We are given a line segment to start, which will become the hypotenuse of a 30-60-90 right triangle. It works by combining two other constructions: A 30 degree angle, and a 60 degree angle. Because the interior angles of a triangle always add to 180 degrees, the third angle must be 90 degrees. A Euclidean construction.

This page shows how to construct (draw) a 60 degree angle with compass and straightedge or ruler. This construction works by creating an equilateral triangle. Recall that an equilateral triangle has all three interior angles 60 degrees. We use one of those angles to get the desired 60 degree result. See the proof below for more details. A Euclidean construction.

Given three points, it is always possible to draw a circle that passes through all three. This page shows how to construct (draw) a circle through 3 given points with compass and straightedge or ruler. It works by joining two pairs of points to create two chords. The perpendicular bisectors of a chords always passes through the center of the circle. By this method we find the center and can then draw the circle. A euclidean construction.

This page shows how to construct a line parallel to a given line that passes through a given point with compass and straightedge or ruler. It is called the 'angle copy method' because it works by using the fact that a transverse line drawn across two parallel lines creates pairs of equal corresponding angles. It uses this in reverse - by creating two equal corresponding angles, it can create the parallel lines. A Euclidean construction.

How to construct a regular hexagon given one side. The construction starts by finding the center of the hexagon, then drawing its circumcircle, which is the circle that passes through each vertex. The compass then steps around the circle marking off each side. A Euclidean construction.

How to construct (draw) a regular hexagon inscribed in a circle with a compass and straightedge or ruler. This is the largest hexagon that will fit in the circle, with each vertex touching the circle. Ina regular hexagon, the side length is equal to the distance from the center to a vertex, so we use this fact to set the compass to the proper side length, then step around the circle marking off the vertices. A Euclidean construction.

Constructing the tangent to a circle at a given point on the circle with compass and straightedge or ruler. It works by using the fact that a tangent to a circle is perpendicular to the radius at the point of contact. It first creates a radius of the circle, then constructs the perpendicular bisector of the radius at the given point.

How to construct (draw) an isosceles triangle with compass and straightedge or ruler, given the length of the base and one side. First we copy the base segment. Then we use the fact that both sides of an isosceles triangle have the same length to mark the topmost point of the triangle that same distance from each end of the base. A Euclidean construction.

This page shows how to construct (draw) the circumcenter of a triangle with compass and straightedge or ruler. The circumcenter of a triangle is the point where the perpendicular bisectors of the sides intersect. It is also the center of the circumcircle, the circle that passes through all three vertices of the triangle. A Euclidean construction.

This page shows how to construct (draw) the incenter of a triangle with compass and straightedge or ruler. The incenter of a triangle is the point where all three angle bisectors always intersect, and is the center of the triangle's incircle. A Euclidean construction.

Given an angle formed by two lines with a common vertex, this page shows how to construct another angle from it that has the same angle measure using a compass and straightedge or ruler. It works by creating two congruent triangles. A proof is shown below. A Euclidean construction