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Because the back of the postcard is exposed, anyone who picks up your postcard will be able to see what you have written. Avoid writing things that you wouldn’t tell a stranger, like personal bank information, intimate secrets, or anything that could be used to steal your identity. Keeping your writing on the left side of the postcard is very important in ensuring that the postcard makes it to its destination. Creeping into the address part of the postcard (the right side) could make the address difficult to read and could throw off the post office. If you have too much to write about, consider sending a letter in addition to the postcard. Keep the message short and brief on the card, and write a longer message in a letter. Write the return address on the top left of the postcard. If you plan to be travelling within a month of sending the postcard, write the return address of your next destination. Return addresses are best if you know exactly where you will be over time. Sloppy or illegible handwriting may result in the post office sending your postcard to the wrong place, or discarding because of bad handwriting. If you are worried about your handwriting, practice writing the address in print on scrap paper before transcribing it to the postcard. Be sure to write legibly for both the recipient address and the return address. The note itself does not have to be too neat, as long as your recipient can read it.

Summary:
Do not write about anything too personal. Do not let your writing cross over to the right side of the postcard. Add a return address if you will be in one place for a while. Write legibly, especially with addresses.