used letterlocking to protect their handwritten correspondence from snoops and spies.

 

A letter from Mary Queen of Scots, sent on Feb. 8, 1587, to Henry III, king of France. National Library of Scotland

What did Catherine de’ Medici, Queen Elizabeth and Mary Queen of Scots have in common? They all used letterlocking to protect their handwritten correspondence from snoops and spies. The 16th-century technique is seen as a precursor to modern encryption. A recent paper details what researchers have learned about the locked letters in two decades of study.