Angles and Lines

What Is an Angle?#

An angle is formed when two rays share a common endpoint called the vertex. Angles are everywhere in our world — in the corners of rooms, the hands of a clock, the blades of scissors, and the pages of an open book.

Parts of an Angle#

• Vertex — the point where two rays meet
• Rays — the two sides of the angle that extend from the vertex
• Opening — the space between the two rays, which determines the angle's size

Measuring Angles: Degrees#

Angles are measured in degrees (°). A full circle contains 360°. This means:

• A half-turn = 180°
• A quarter-turn = 90°

The more 'open' an angle is, the greater its degree measure.

Types of Angles by Degree#

Acute angle: Measures less than 90°

• Example: The angle of a pizza slice, a sharpened pencil tip
• It is a 'small' opening

Right angle: Measures exactly 90°

• Example: The corner of a square, a book on a table
• It is marked with a small square symbol at the vertex

Obtuse angle: Measures more than 90° but less than 180°

• Example: The opening of a book, a reclining chair
• It is a 'wide' opening

Straight angle: Measures exactly 180°

• It forms a straight line
• Example: A flat table surface

Quick Memory Trick#

• Acute = A small opening (A looks like a sharp peak)
• Obtuse = Open wide (O is round and open)
• Right = Rectangular corner (R stands for right angle)

Learning to recognize angle types at a glance will help you quickly classify and work with geometric shapes.