Each day, he sent to her house, on Dupont Circle, a packet of billets-doux and state documents.

NEWYORKER: State of the Union

Young lovers in Victorian England, forbidden to express their affection in public and fearful that strict parents would intercept their billets-doux, sent coded messages through the personal columns in newspapers.

FORBES: I've got a secret

Despite the growing tempo and pervasiveness of hostile intelligence operations within U.S. borders, they have downgraded counterintelligence across the board in dollars, billets, priority and station including their handling of the largest peacetime espionage ring in U.S. history.

WSJ: Michelle Van Cleave: Russian Spies Haven't Gone Away