Lines, Line Segments, Rays, and Angles

Welcome to geometry! To understand shapes, we first need to learn about the basic building blocks used to make them: lines, rays, and angles.

Points, Lines, Line Segments, and Rays

Everything in geometry starts with a point.

• Point: An exact location in space. We usually draw it as a small dot.
• Line Segment: A straight path between two points. It has a beginning and an end. The two points on the ends are called endpoints.
• Ray: A straight path that starts at one endpoint and goes on forever in one direction. Think of a sunbeam (a ray of light) starting at the sun and traveling through space.
• Line: A straight path that goes on forever in both directions. It has no endpoints. We draw arrows on both ends to show it never stops.

What is an Angle?

An angle is formed when two rays or line segments meet at the same endpoint. That shared endpoint is called the vertex.

Angles are measured in degrees (written with the symbol $^\circ$).

Types of Angles

There are three main types of angles you need to know:

1. Right Angle: This angle forms a perfect square corner. It measures exactly $90^\circ$. If you look at the corner of a book, a piece of paper, or a rectangle, you are looking at a right angle.
2. Acute Angle: An angle that is smaller or "sharper" than a right angle. It measures less than $90^\circ$. (A fun way to remember: think of it as a "cute" little angle!)
3. Obtuse Angle: An angle that is larger or wider than a right angle. It measures more than $90^\circ$ but is not a completely straight line.

Let's Practice!

Here are a few questions to test your new geometry skills:

• Question: How many endpoints does a line segment have?
  • Answer: Two.
• Question: How many right angles are inside a standard rectangle?
  • Answer: Four. Every corner of a rectangle is a right angle.
• Question: If an angle is open wider than the corner of a piece of paper, what type of angle is it?
  • Answer: An obtuse angle.