Informal Invitations Etiquette

Written on personal notepaper or on an informal or correspondence card, informal invitations are nonetheless written in the third person but are less structured in form than truly formal invitations.
Informal invitations come in various forms. For instance, you can send a fill-in invitation that you buy in a card shop. You can write on informal notepaper or on a folded note with a monogram. On a note with a monogram, start writing on the front if the monogram is placed to one side; start inside, under the fold if the monogram is in the middle.
You can reply by phone if the invitation includes a telephone number. Otherwise, respond on your own stationery—either plain, informals, or correspondence cards. If the invitation says “Regrets Only,” you need not respond if you can attend.
However, it’s still a good idea to let the host know you’re planning to attend.