Basic Terms and Definitions

• A part or portion of a line with two endpoints is called a line segment.
• A part of a line with one endpoint is called a ray.
• If three or more points lie on the same line, they are called collinear points;
• otherwise, they are called non-collinear points.
• An angle is formed when two rays originate from the same endpoint.
• The rays making an angle are called the arms of the angle and the endpoint is called the vertex of the angle.

Linear Pair Axiom

1. If a ray stands on a line, then the sum of two adjacent angles so formed is 180°.
2. If the sum of two adjacent angles is 180°, then the non-common arms of the angles form a line.