For a standard R.S.V.P., the company's address, zip code and telephone number should be included in the information. If you request only a telephone reply, it is a good idea to include the name of the person in the organization to whom the replies should be made. Moreover, a person who is not planning to attend is probably the least likely to make the extra effort to call or write, making it more difficult for you to obtain an accurate count. Replies to formal invitations should be made within two days. They should be handwritten in the third person on white or ecru letter sheets with black ink.
Acceptances repeat the event, the date, and the time.
Regrets repeat the event and the date, and give a brief reason for declining. The wording of an informal reply is dictated by the invitation and the relationship to the host or hostess. Informal replies can be written on correspondence cards, monogrammed notes, or calling cards. When declining, a reason must be given. Regardless of how people reply, it's a good idea to get a telephone number, according to Kahan, because you never know what will happen at the last moment.