Edit surface symmetrically

Rhino 7:

Sorry, to be ignorant. I have noticed, that tutorials often edit, say, a surface symmetrically; when a control point in one side is moved, the corresponding mirror point moves symmetrically.
However, I cannot figure out how to switch that function on. Would some one be so kind as to give me a hint?

Thank you in advance.

Best regards,

Hi Neils - in V7 you can do this on SubD objects only, using the Reflect command. For surfaces, the best you can do is Symmetry, applied, with History recording on, to a single surface that is then mirrored across the symmetry plane - editing the original will update the symmetric copy.