During World War II, the military and Post Office Department used this method to reduce the bulk of mail, conserving badly-needed space. The 37 mail bags required to carry 150,000 one-page letters could be replaced by a single mail sack.

On the envelope, notice that “···—” symbol? That's Morse code for “V” (for Victory). The “Passed By” stamp meant the letter—from Army Sgt. Bill Ingram to his daughter Patsy—had been read by censors, and any vital information, such as troop movements or other military secrets, had been blacked out, just in case Patsy was a spy.