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A [[triangle]] with two sides of equal length.

An '''isosceles triangle''' is a [[triangle]] with exactly two [[congruent]] sides. The [[angle]]s opposite the sides of equal length are equal in measure. The term is originally Greek, meaning "equal legs," since the two sides of equal length are usually referred to as the "legs" of the triangle, with the remaining side being called the "base." In Euclid's ''Elements'', triangles which are not isosceles were referred to as ''cholos'', meaning "lame."