Every other day, when Janie Bielefeldt writes to her husband, who is deployed in Afghanistan, she places her stamps upside down and diagonally on the letters as a way to say "I miss you." Susan Haggerty says "I love you" by putting her stamps upside down on letters to her son, stationed in Iraq.

Noma Byng does the same thing with the letters she sends to her husband when he is serving abroad as a way of trying to convey what words cannot. "You do everything you can to make the letters seem like more than a piece of paper," Mrs. Byng said.

For most people, the front of an envelope is simply a place for addresses and postage, and a crooked stamp indicates little more than that the sender was in a hurry. But for others, this tiny sliver of real estate is home to a coded language, hidden in plain sight, that has been passed down through the generations for more than a century.

"Another military wife told me that her grandmother used to flip her stamps when writing her husband, who was deployed overseas," said Mrs. Bielefeldt, an ex- marine living in Jacksonville, N.C. "It's just something you hear about on the base."