Erecting equilateral triangles on the sides of a triangle internally leads to the inner Napoleon triangle, while doing so externally gives the outer Napoleon triangle.
Triangle Triangle Erecting
See also
Inner Napoleon Triangle, Outer Napoleon TriangleExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Triangle Triangle Erecting." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/TriangleTriangleErecting.html