Base (Isosceles Triangle)
High School
Definition
An isosceles triangle's base is the side that is not congruent to the other. Remember that an isosceles triangle has two equal sides (called legs) and its last remaining side is called the base. Another way to phrase this is that the base of an isosceles triangle is the side that is not the legs of the triangle.
Worked examples
\(\triangle ABC\) with \(AB = AC = 5\) cm and \(BC = 8\) cm. Base = \(BC\)
The two equal sides (legs) are AB and AC; the remaining side BC is the base.
\(\triangle DEF\) with \(DE = DF = 7\) in. Base = \(EF\)
Since DE and DF are congruent, EF is the base of this isosceles triangle.
Common mistakes
- Calling any side the base without checking which sides are congruent → Identify the two equal legs first, then the remaining side is the base The base is defined by which side is different, not by position or orientation.
- Assuming the base is always horizontal or at the bottom → The base is the non-congruent side, regardless of how the triangle is drawn An isosceles triangle can be rotated; orientation does not determine the base.
Where you'll use it next
You'll use the base when finding the area of an isosceles triangle, proving the base angles theorem, and identifying congruent parts in triangle proofs and constructions.
Found in 1 StudyPug lesson
Mastering Triangle Classification: Your Key to Geometry Success
10th Grade10thGeometry
Unlock the world of geometry by mastering triangle classification. Learn to identify and understand various triangle types, their properties, and real-world applications. Boost your problem-solving skills today!
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026