Line (geometry)

What Is a Line?

A line is a very special shape in geometry. Imagine a very thin, straight string that goes on and on forever in both directions. It has no width or thickness — it is just a straight path.

Where Do We See Lines?

We can think about lines in different places. On a flat piece of paper, a line lies in a space with two directions — up and down, left and right. In our world, lines can also go forward and backward, so they live in a space with three directions. Even though real things like rulers are not perfect, they help us picture lines.

Why Are Lines Important?

Lines are important because they help us understand shapes and spaces. Ancient mathematicians, like Euclid, wrote about lines a long time ago. They described a line as having “length but no width.” Today, lines are a basic idea used in many kinds of math.

Fun Parts of a Line

Sometimes we talk about smaller parts of a line:

• A ray starts at a point and goes forever in one direction.
• A line segment is a piece of a line with two ends.
• A number line is a line that shows numbers, with positive numbers to the right and negative numbers to the left.