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See a common question answered below by Emily Post
Thank-you Note Stationery
Q. I have note cards with my
married initials on them. Should I use these to write thank-you notes
for my shower gifts or should I wait until I am married and use
something else in the meantime?
A. Wait until you are married
before you break out the stationery monogrammed with your married
initials. Fold-over note cards are fine to use for shower thank-you’s.
You can use your monogrammed maiden-name stationery, too.
There is no single stationery required for thank-you notes, although
you’ll probably use a standard one-sided or single-fold note card and
matching envelope. The paper can be plain or bordered, white, ivory,
ecru or a pastel color. Use ink that is easy to read; black ink is
always legible.
A bride signs with her maiden name (or pre-marriage name, if an
encore bride) before the wedding, and signs her married name afterward.
When using monogrammed stationery, the notes sent by the bride before
the wedding have her maiden name initials; post-wedding notes have her
married initials or the couple’s last-name initial.
Grooms can write thank-you notes, too! When husbands and wives share
monogrammed stationery, the last/married name initial, hyphenated
initials, or double last-name initials (when the wife keeps her maiden
name) are used.