Geometry is a branch of mathematics, just as numbers are the basic building block of algebra, so geometry has its basic building block, which is the point.

The point is the basic component of the line segment, ray, line and all geometric shapes.

In this lesson, we will try to define the concept of line segment, line, and ray, with mention of their mathematical symbols.

We will also see the relative positions of two lines in the plane: parallelism, perpendicularity and intersection.

The point is the basic structural unit in geometry, and it has **no dimensions**, and we often represent it as a single point with the tip of a pen or a circular point using a computer.

A Line is an infinite set of compact collinear points, and it has **one dimension**.

Geometrically, we take two different points on this line so that both sides are not bound by these two points.

In the above figure, the line is denoted by the symbol: or

In some countries, the Line is denoted by the symbole or .

The Ray is part of the line, with one endpoint, and the other side is unfinished, so it starts at one point and goes on forever in some direction. And it has one dimension.

Geometrically, we take two different points so that the first point is the endpoint and the second point is on the infinite side.

In the above figure, the ray is denoted by the symbol:

In some countries, the ray is denoted by the symbole .

The line segment is a part of the Line so that it have two endpoints, that is, its length is finite. And it has one dimension.

Geometrically, we take two different points so that the first point is on the first endpoint and the second point is on the second endpoint.

In the above figure, the line segment is denoted by the symbol: or

In some countries, the Line is denoted by the symbole or .

A plane is a flat surface such that if we take two points from this plane, we always get a straight line belonging to this plane.

In the above figure, the plane is denoted by the symbol:

We say that two lines are parallel if they have no common point, that never intersect.

The following Lines are parallel:

In the above figure, the parallel lines denoted by the symbol: or

We say that two lines intersect if they share only one point.

The following Lines are intersect at point I:

We say that two lines are perpendicular if they intersect and form a right angle at their point of intersection.

The following Lines are perpendicular:

In the above figure, the intersecting lines denoted by the symbol: or

We say that three or more points are collinear if they belong to the same line.

In the example below, the points A, B and C are collinear:

But, in the example below, the points A, B and C are non-collinear: