A scalene triangle is a triangle whose three sides all have different lengths. Because no two sides match, no two of its angles are equal either. A scalene triangle can be acute, right or obtuse: the 3-4-5 right triangle is scalene, and so is a triangle with sides 4, 6 and 7.
Different numbers of tick marks show three different side lengths. The biggest angle (86°) sits across from the longest side (7).
Formal definition and notation
Triangle ABC is scalene when AB, BC and CA are pairwise unequal. Since a larger side is always opposite a larger angle, its three angles are also pairwise unequal. Its sides must still satisfy the triangle inequality, and its angles still add to 180°.
✓ InsteadA 3-4-5 right triangle has three different sides, so it is a right scalene triangle.
Scalene describes the sides; right describes an angle. A triangle gets one name from each list.
✗ MistakeCalling a triangle with sides 5, 5 and 8 scalene because it looks lopsided.
✓ InsteadTwo sides are 5, so it is isosceles.
Classify from the given lengths or tick marks, not from how the drawing looks.
✗ MistakeAny three different lengths, like 2, 3 and 9, make a scalene triangle.
✓ InsteadCheck the triangle inequality first: 2 + 3 = 5, which is less than 9, so no triangle exists.
The two shorter sides must add to more than the longest side, or they cannot meet.
Not to be confused with
Scalene Trianglevs.Isosceles triangle
An isosceles triangle has at least two equal sides and two equal base angles, such as 5, 5, 8. A scalene triangle has none.
Scalene Trianglevs.Equilateral triangle
An equilateral triangle has three equal sides and three 60° angles, the opposite extreme from scalene.
Quick check: 3 questions
Question 1 of 3
Which side lengths make a scalene triangle?
Show answer and explanation
Answer: D. 6, 8 and 10 are all different (and 6 + 8 is more than 10). Choices A and C have two equal sides (isosceles), and B has three (equilateral).
Question 2 of 3
Can a scalene triangle have a right angle?
Show answer and explanation
Answer: B. 5, 12 and 13 are all different and 25 + 144 = 169, so it is a right scalene triangle. C describes an isosceles right triangle and D an equilateral one.
Question 3 of 3
A scalene triangle has angles of 30° and 70°. What is the third angle?
Show answer and explanation
Answer: C.180 − 30 − 70 = 80. Choices A and B repeat an angle, which a scalene triangle cannot do, and D is the sum of the two given angles.
Scalene Triangle: frequently asked questions
Can a scalene triangle be obtuse?
Yes. A scalene triangle can be acute, right or obtuse. Sides of 3, 5 and 7 make an obtuse scalene triangle, because 32 + 52 = 34 is less than 72 = 49, so the angle across from the 7 is greater than 90 degrees. The name scalene only says that all three sides differ.
Does a scalene triangle have a line of symmetry?
No. A line of symmetry would fold one side onto another side of equal length, and a scalene triangle has no two equal sides. An isosceles triangle has one line of symmetry and an equilateral triangle has three, but a scalene triangle has zero.
Updated November 2026·Reviewed by the Effortless Math team·Standards: CCSS 4.G.A.2, CCSS 7.G.A.2
Cite this entry
Effortless Math Team. “Scalene Triangle.” Effortless Math Dictionary. Updated November 2026. https://www.effortlessmath.com/math-dictionary/scalene-triangle/
Flashcard
TermScalene Triangleno two sides equal
DefinitionA triangle with three different side lengths and three different angles, such as sides 4, 6 and 7, or a 3-4-5 right triangle.
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