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Before you address envelopes for any formal event such as a wedding, charity benefit, bar/ bat mitzvah, quinceañera, you should check all of the address and title information for each person.  Handwrite or print your envelopes. Hiring a calligrapher, or a person professionally trained to artistically hand-letter documents, is also an option to address your envelope. Envelopes hand-written by yourself or a calligrapher in a dark-colored ink are the preferable option for formal, non-business events.  Buy high quality paper and envelopes that either come as a set or match to help convey the formality of the event. Remember that this is an envelope for a formal event: spell out every word. Don't abbreviate anything other than "Mr.", "Mrs.", or "Ms." How you address the envelopes will vary based on their marital and/ or professional status.  Address women according to their marital status or their professional titles. Married women often use "Mrs." In some cases, the person might prefer "Ms." For divorced women or women over the age of 18, "Ms." is an excellent option. For younger women, you may also use "Miss." For example: "Mrs. Charlotte Smith," "Ms. Lily Smith." Address all men by their names preceded by "Mr." For example: "Mr. John Smith." If you are addressing an envelope to a man with the same name as his father, or his son, include "Jr." or "Sr." at the end of each name, respectively. For example: "Mr. Christopher Smith, Jr." or "Mr. Christopher Smith junior". If a man has the same name as his father and grandfather and is considered "the third" or beyond, use Roman numerals to address him. For example, "Mr. Christopher Smith IV." Address couples based on their marital status. Addressing unmarried couples is different from how you address married couples. Address married couples as "Mr." and "Mrs." followed by the man's name. For example, "Mr. and Mrs. Christopher Smith." Address unmarried couples who live together by their names preceded by their appropriate titles. For example, "Ms. Jane Doe" and "Mr. John Smith." Address men and women with their professional titles when applicable. Address the envelope with their title, which does not require that "Mr.," "Mrs.," "Miss," or "Ms." precede it. Different titles people may use include "Dr." "Reverend" or "Judge." If you are not sure of a person's formal title and cannot locate this information, a good rule of thumb to use is to "elevate" the position you think they have. For example, if you're not sure someone is a Captain or General in the military, write their title as "General." This way, you won't offend anyone. The following is a list of titles you may encounter in addressing your formal envelope:  Include the names of children on envelopes at your discretion. If children are not invited to the event, do not include them on the envelope. If you are inviting children, write only their  first names on the second line underneath their parents. Write this information just under the names of persons, including children on the envelope. As with names and titles, don't abbreviate with the address. Write out words like "street," "avenue," or "circle." For example: "200 Main Street," "15 Dupont Circle," "1600 Pennsylvania Avenue, Northwest." For example, "Washington, DC 20002," or "Berkeley, CA 94703."  If you're not sure of a ZIP code, you can look it up on the website of the United States Postal Service.  For a foreign country, it's useful to look at formatting conventions for international addresses.

Summary:
Verify information. Write the names of your guest or guests on the first line of the envelope. Add the address on the second line. Address the final line of a formal envelope with the city, state, and ZIP code.