What is a triangle?

A triangle is a flat shape with three straight sides and three angles. Its three inside angles always add up to 180°. Triangles are named by their sides: equilateral (all three sides equal), isosceles (two sides equal) and scalene (all sides different).

Examples

• An equilateral triangle has 3 equal sides and 3 equal 60° angles.
• A triangle with a right angle is called a right-angled triangle.
• The three angles of any triangle add up to 180°.

Quick facts

• 'Tri' means three, so a triangle has three sides.
• A triangle is the simplest polygon: no flat shape can have fewer than three straight sides.