Fake mirror of mesh

Hi guys, I forgot how to make a fake mirror of mesh.
Specifficaly, If I got one left/right side of face, how to mirror that face, so I can see what the final mesh will look like so when moving vertex on one side it reflects on the other.
Hope I made question all right, I can’t remember how to doit in blender.

Try adding a mirror modifier?

AAAaaahhhh yesyesyes indeed my dear Sherlock, as you sead: apart from from hidden, everithing else is obvious. :smiley:
Thanks alot man.