How did people in hiding communicate with confidants before modern means of communication?

In the 17th century it was practically impossible to hide without at least one person knowing where you were – even Jesuits in priestholes needed food and water. A prisoner might have short messages smuggled into gaol hidden in an egg, while longer missives could be folded into packages small enough to secrete in a lady’s wig.

In a society more porous than today’s, outgoing letters could be delivered by private bearer (perhaps accompanied by an oral message), or via clandestine networks of individuals who could travel without suspicion, such as apothecaries and nurses. Letters might also travel through more official channels to neutral destinations to an individual using a codename (Susan Hyde, a royalist spy during the Interregnum, used “Mrs Simburbe”), or from where an enclosure, a letter hidden within a letter, could be forwarded.

Before the 19th-century invention of the modern gummed envelope, letters formed their own packaging, a folding technique known as “letterlocking”. Some “locks” were highly elaborate, rendering a letter impossible to open without damaging it – Charles I recognised letters from one of his female spies “by their folding” alone. Cromwell’s spymaster John Thurloe had a “Black Chamber” dedicated to intercepting, opening, transcribing and resealing letters before sending them on their way.

Even if intercepted, a letter might hide its secret meaning behind substitution ciphers or its secretive nature with invisible inks or mercantile discourse (“our trade is slow” meant “we are being watched”). Keeping communications secret was no easy task, but it could be the difference between defeat and victory.