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Three sides with different roles
The hypotenuse is opposite the right angle. The other two sides are legs. Relative to a chosen acute angle, one leg is adjacent and the other is opposite.
Leg² + leg² = hypotenuse²
Worked example
In a 3-4-5 triangle, 3 and 4 are the legs and 5 is the hypotenuse because 3² + 4² = 5².
Why the labels matter
Pythagorean-theorem problems mainly require identifying the hypotenuse. Trigonometry problems also require identifying the opposite and adjacent sides relative to the chosen angle.
Common mistakes
- Calling the longest side a leg.
- Using opposite/adjacent labels without specifying the angle.
- Applying a right-triangle formula to a non-right triangle.
Frequently asked questions
What are the two shorter sides called?
The legs.
What is the side opposite the 90° angle?
The hypotenuse.
Can opposite and adjacent switch?
Yes. They depend on which acute angle you are using.