party invitations the formal kind

[roozle]

Recently my LJ has had some musings about the etiquette of invitations, social inclusivity vs the desire for small events and the like.  Most of the time those are pretty informal things, and invitations by email is what we mostly discussed.  But I was thinking about the details of formal invitations and who's included in those.

I'm curious about the following:
Poll Wedding invitations and bringing a date 
Anyway, wondering what the hive mind thinks.  Edited to add: No actual real world invitations were  harmed consulted in the making of this poll.  All in the hypothetical realm.