bent wrote:
not clear to me, show me

understand cut square tight
understand 45 mitre
understand double 45 mitre and comes to point. Q) are we talking equal and
very pointy point?

Q) cut off mitre face /undercut , and cut off mitre face, respectively for
the 45, & double 45 (sorry I'm Canadian & therefore slightly French; hence
colour?):
in the first case, is cut off mitre face /undercut 2 steps, possibly at the
same time, and then are we talking exactly the same step(s) in the second
case

Q) I don't understand cut off mitre face /undercut; is this a curved cut?
I'm thinking....

It sounds like coping is new to you. Here's a primer on cope cuts:
http://www.nwrenovation.com/12insidemoldingcorner.html

Does that help?

-Mike