Stamp messages are decoded by a mutual understanding of what the placement of the stamp indicates. For example, a stamp turned upside down carries the message of "I love you" unless it's an American flag stamp, in which case it is often a sign of protest about a current war, according to Ian Urbina in the New York Times.
Stamps tilted to the left have various meanings, depending on the origin of the letter, according to Urbina. In England, a stamp tilted to the left means "Will you be mine?" In Germany, the same placement means "Why don't you answer?"
The practice of secret stamp messages started in the Victorian era when letters were often monitored by parents to avoid unapproved courtships, Urbina writes. It continued during war times due to the reading of military mail by inspectors protecting against classified information being passed through the mail, states Stamp Collecting Resource. Secret stamp codes also were used for postcards sent to loved ones, because the lack of an envelope allowed anyone to easily read messages.
This practice returns in times of war when loved ones are apart for long periods of time without electronic communications. It provides a way of adding a little extra care and affection to letters exchanged between soldiers and their loved ones at home, Urbina suggests.