Line Segment is the portion of the line between two points A and B. A line segment has two end points. Ray is a line segment AB when extended in one direction.

An infinite number of lines can be drawn through a point. All lines are called **concurrent lines**. One and only one line can be drawn passing through two given points.

If a line can pass through three or more points, then these points are said to be **collinear points**, otherwise points are non-collinear.

Two lines are called **parallel lines** if both have no points in common or if the distance between the lines is same everywhere.

**Angle** is formed by two rays with a common initial point called vertex and measured in degrees.

**Acute angle:**An angle whose measure is less than 90º.**Right angle:**An angle whose measure is 90º.**Obtuse angle:**An angle whose measure is more than 90º but less then 180º.**Straight angle:**An angle whose measure is 180º.**Reflex angle:**An angle whose measure is more than 180º and less than 360º.

Two lines or rays making a right angle with each other are called **perpendicular lines**.

**Complementary angles:** Two angles are complementary if the sum of their measures is 90º.

**Supplementary angles:** Two angles are supplementary if the sum of their measures is 180º.

**Adjacent angles:** Two angles having a common vertex, a common arm and non common arms on opposite sides of the common arm.

When a transversal intersects two parallel lines, then

- each pair of corresponding angles are equal.
- each pair of alternate angles are equal.
- each pair of interior angles on the same side of the transversal are supplementary.

For example:

- ∠2 = ∠6, ∠3 = ∠7
- ∠3 = ∠6 and ∠4 = ∠5
- ∠3 + ∠5 = 180º and ∠4 + ∠6 = 180º