Reflections
Opposite Isometries
Any isometry that reverses the orientation of a triangle is called an opposite isometry. When you read off the vertices of triangle ABC in cyclic order, they are read in a counterclockwise direction. When you read off the vertices of triangle A'B'C' in cyclic order, they are read in a clockwise direction.
is the notation we will use for a reflection in line m.
In the sketch below, we would write .