 The two angles opposite the legs are equal and are always acute, so the classification of the triangle as acute, right, or obtuse depends only on the angle between its two legs.Euclid defined an isosceles triangle as a triangle with exactly two equal sides, but modern treatments prefer to define isosceles triangles as having at least two equal sides.The difference between these two definitions is that the modern version makes equilateral triangles (with three equal sides) a special case of isosceles triangles.