Two letter monogram- which letters?? Invite wording calamity.

Am I functionally moronic? I can't figure this out.

I am not taking FH's last name. On our invites, I want to put a two letter monogram. Do we use our last name initials, or our first name initials?

As I read that question, I become mortified, as I think it's obvious that it should be the last name initials, but I'm so scared to put in the final word on our proof and have it be wrong.

ALSO, if it's Mr. and Mrs. XYZ and Mr. and Mrs. ABC request the honor of your presence at the wedding of their children, do you use the children's last names? I was doing
Mr. and Mrs. XYZ
and
Mr. and Mrs. ABC
request the honor of your presence
at the wedding of their children
Sue Mary XYZ
and
John Robert ABC.

Do I leave out OUR last names, because the parents last names are already there, and it's implied?

Thank you in advance for not calling me Moron in your responses. It's way too early.

i would just use the 2 first name initials if you arent taking his last name...jmo

2 - if you are writing the parents last name and you have the same last names then you leave them out