What Is an Angle?
Before we explore different angle types, let's make sure we're clear on what an angle actually is.
An angle is the amount of turn or rotation between two rays (lines) that share a common endpoint called the vertex.
Angles are measured in degrees (°). A full turn is , a half turn is , and a quarter turn is .
You already encounter angles everywhere — when you open a book, when the hands of a clock move, or when two roads meet at a corner. Understanding angles is the foundation for everything we'll learn in this subtopic.
Think of an angle like opening a door. A door fully closed is , slightly open might be , and wide open against the wall is .
Quick reminder of angle categories by size:
- Acute angle: less than
- Right angle: exactly
- Obtuse angle: between and
- Straight angle: exactly
- Reflex angle: between and
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