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An isosceles triangle is a triangle in which at least two edges have equal length. Equivalently, an isosceles triangle is one in which at least two angles have equal measure. Equilateral triangles are a special class of isosceles triangles in which all three sides and angles are equal.
Every isosceles triangle has reflectional symmetry. The axis of symmetry passes through the vertex which is shared by the edges of equal length, and it is also the triangle's median, altitude, and angle bisector of that vertex.