Some countries require that you put a floor number in the return address when you are sending a package or letter. It is not required to place the floor number on your address in the United States. One country that requires the floor number is Argentina. It helps the postal service find an individual quickly and more accurately if you include the apartment or office number, which incorporate the floor number. It is also preferred that the return address be placed on the back of the envelope.
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Select the size envelope that will accommodate your letter.
Turn the envelope over to the back, where you will write the address.
Format the address on the center of the back of the envelope. In the upper left-hand corner place the return address. Start with your name or business name on the first line. On the second line, add the street number and street address followed by a comma and then the floor number or apartment number. The third line contains the postal code along with the city, town or locality. Write the country name on the last line of the address, if applicable.