(i) Given two points P and Q on a line the segment of line with end points P and Q, is called the line segment PQ. The line segment PQ has fixed length. Line segment PQ is denoted by PQ.

(ii) Three or more points P, Q and R are said to be collinear if they lie on a single straight line .

(iii) Parallel lines are lines in a plane which do not meet; that is, two lines in a plane that do not intersect or touch each other at any point are said to be parallel lines. Parrallel lines are denoted by

(iii) Two lines are intersecting if they have a common point. The common point is called the point of intersection.

(iv) When three or more lines in a plane or higher-dimensional space are said to be concurrent if they intersect at a single point.

(v) A ray is a line with a single endpoint (or point of origin) that extends infinitely in one direction.

(vi) If P, Q and R be the points on a line , such that Q lies between P and R, and if we remove the poiny Q from the line the two parts of the line that remains are each called a half line.