Three mirror lines make a triangle.

For each corner we are free to choose the rotational symmetry:

center | (the corner at the left) | |

left | (as seen from the center) | |

right | (as seen from the center) |

sum of angles = ^{o}

If the sum of the corner angles is not 180^{o} degrees,
then one of the mirror lines has to be a circle arc!

**What is the mirror image at a circle arc?**