# Line

In geometry, a **line** is a continuous one-dimensional entity that has length but no width or depth.

It is one of the fundamental concepts in geometry.

Lines are composed of an infinite sequence of points arranged in a continuous manner.

If the continuous movement of points occurs on a plane, the line is called a **plane line**.

If the line consists of points on different planes, it is called a **skew line**.

A line can be bounded by two **endpoints** A and B, have only one endpoint A, or have no endpoints at all.

If the endpoints coincide, it is called a **closed line**; otherwise, it is an **open line**.

A closed line divides the plane into two regions:

**Interior points**

These are the points that lie on the segments formed by joining any two points on the line.**Exterior points**

These are the points that belong to the plane but are not on the segments between two points of the line.

An open or closed line is called an **intersecting line** if it passes through the same point multiple times; otherwise, it is called a **simple line**.

If all its points are aligned, the line is called a **straight line** if it has no endpoints, or a **segment** if it is bounded by two endpoints.

In all other cases, the line is called a **curve** or simply a **curved line**.

A curve bounded by two endpoints is called an **arc**.

Therefore, straight lines, rays, and segments are specific cases of lines.

A line composed of a sequence of segments in different directions is called a broken line or polygonal line.

**Broken line**

A sequence of straight segments, called sides, each connected at one endpoint to the next segment. A broken line does not need to close on itself. In other words, a broken line can be open or closed.

**Polygonal line**

A closed broken line, meaning the last segment ends where the first segment started. Therefore, a polygonal line is always closed.

If there are no intersections, the closed polygonal line is called a__polygon__.

Essentially, the main difference between the two is that a broken line can be open or closed, while a polygonal line is always closed.

**Note**: In Euclidean geometry, a line is often defined as the shortest path between two points. However, this concept is more accurately termed **distance**. It is the segment that connects any two points on the curve.

Among any distinct points A and B on the curve, there are infinite curves but only one straight line. The distance is the segment of this straight line bounded by the two endpoints A and B.

And so on.