Such folded letters were common before the invention of the envelope. Letters were folded multiples times with tucks or slits and often sealed with wax. The paper they are written on is now so fragile that opening them up can damage them.
“Studying folding and tucking patterns in historic letters allows us to understand technologies used to communicate,” says Jana Dambrogio at the Massachusetts Institute of Technology. Now, her team is able to reconstruct what is inside without damaging the letters.