**Pairs Of Lines**

There are four pairs of lines:

- Intersecting lines
- Parallel lines
- Skew lines
- Perpendicular lines

If we draw two lines in a plane surface, they will either intersect each other at a point or never meet each other at all.

**Intersecting lines**

In figure, lines AB and CD intersect each other at a point (O), such lines are called intersecting lines.

**Parallel lines**

In figure, lines AB and CD never meet each other at any point, i.e., they are equidistant from each other at each and every point. Such lines are called parallel lines. Parallel lines AB and CD are

represented as AB || CD. Here, symbol ‘**||**’ is read as ‘**is parallel to**’.

**Skewed lines**

Two lines in two different planes that are neither parallel nor intersecting are called skewed lines.

In figure, l and m lie in different planes, so these are called skewed lines.

**Perpendicular lines**

Two lines in a plane, which intersect each other and form an angle of 90°, i.e., right angle are called perpendicular lines.

In figure, two lines AB and CD intersecting each other form an angle of 90° at point O. So, AB and CD are perpendicular to each other and is written as AB ⊥ CD. Here, symbol ‘**⊥**’ is read as ‘**is perpendicular to**’.

**Concurrent lines**

Two or more lines that pass through the same point are called concurrent lines and the point of intersection is called the point of concurrency.

In figure, lines l, m, n, and o are concurrent at the point A.