The locus of oppositely and infinitely travelling two points from a position on a plane is called a straight line.

Straight line is a basic geometrical concept and it plays vital role in studying the geometry. It has length but infinite and it does not have width, thickness and curvature.

- $P$ and $Q$ are two points on a plane and they are at same location initially.
- The point $P$ is started travelling in left side direction from its initial position and continues travelling infinitely in the same direction. Simultaneously, the point $Q$ is also started travelling in right side direction from its initial position and continues travelling infinitely in the same direction.
- The locus of these two infinitely and oppositely travelling points form a straight path and it is called straight line.

A straight line is represented in mathematics based on its geometrical formation.

- Write the names of two points which involve in forming a straight line but name of left point first and then name of right point next.
- Display a Left-Right arrow $(\longleftrightarrow)$ symbol over the names of both the points to represent oppositely and infinitely extended path symbolically.

Here, the points $P$ and $Q$ formed a straight line. So, the points $P$ and $Q$ are written as $PQ$ firstly and then draw left-right arrow over the $PQ$. It means $\overleftrightarrow{PQ}$ is the symbolic representation of a line in mathematics.

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