How do I mirror/symmetry changes?

How do I create symmetry in SubD? e.g. In the model image I inserted an edge as indicated. I want a similar edge on the opposite side. Also, I will want to slide them in unison till I find the correct position. Is this possible?

I kno some programs have built-in mirror or symmetry, Blender I think has it.

Hi @CalypsoArt
The command you are looking for is called Reflect
HTH, Jakob

The important question is why? With NURBS it’s called mirror or symmetry. Do we need another name for the same thing?