EXPLAINER · QUICK READ

Classification of Triangles

5 min readGrade 6 · Geometry

Triangles are everywhere, and they come in different shapes. By classifying them—by side lengths or by angles—we can quickly describe what we're looking at and predict how they'll behave.

By side lengths

Equilateral: all three sides equal (all angles are 60°). Isosceles: two sides equal (the two angles opposite the equal sides are also equal). Scalene: all three sides different (all three angles different).

By angles

Acute: all three angles less than 90°. Right: one angle is exactly 90°. Obtuse: one angle greater than 90°. Note: a triangle can be only one of these—the angles always add up to 180°.

Both at once

A triangle can be classified two ways at once. For example: 'right isosceles' (one right angle, two equal sides). 'Acute scalene' (all angles less than 90°, all sides different). This extra information helps us understand the triangle fully.

KEY VOCABULARY

Equilateral triangle — A triangle with all three sides equal in length.

Isosceles triangle — A triangle with exactly two sides equal in length.

Scalene triangle — A triangle with all three sides different in length.

Acute triangle — A triangle where all three angles are less than 90°.

Right triangle — A triangle with one angle of exactly 90°.

Obtuse triangle — A triangle with one angle greater than 90°.