Skill: Classify triangles informally

A triangle is a shape with three straight sides and three vertices (corners). We can classify, or group, triangles by their attributes:

• Equal sides: Look for sides that are the same length. Matching tick marks show equal sides.
• Right angle: Look for a small square in a corner. That corner is a right angle.
• A triangle can fit more than one classification.

Worked example

Imagine triangle $(ABC)$:

• Sides $(AB)$ and $(BC)$ have matching tick marks.
• Corner $B$ has a small square.

Step 1: Check the shape.
It has three straight sides and three vertices, so it is a triangle.

Step 2: Check the side lengths.
The matching tick marks show that $(AB)$ and $(BC)$ are equal. Therefore, it is a triangle with two equal sides, often called an isosceles triangle.

Step 3: Check the corners.
The small square at $B$ shows a right angle. Therefore, it is also a right triangle.

Conclusion:
Triangle $(ABC)$ is both isosceles and right. Classifications can overlap. Turning the triangle or making it larger would not change its classifications.