WHAT IT IS
Its own envelope
MIT Libraries describe letterlocking as folding and securing a flat sheet of paper so that it becomes its own envelope.
See the numbersFor centuries, a letter locked itself shut.
Before mass-produced envelopes arrived in the 1830s, most letters were folded and secured to become their own envelopes. Researchers call it letterlocking, and it was everyday practice across cultures and social classes.
- Fold the sheet small
- Lock it with paper
- Seal it with wax
- One sheet, no envelope
THE SHORT ANSWER
Paper, folded
into a lock.
Letterlocking turned one flat sheet into a closed packet.
The folds themselves are now historical evidence.
HOW IT WORKED
Folds, tucks and slits
Researchers sort the methods by what was done to the paper: folds or rolls, tucks, slits or holes, adhesive and locks. Together these set a letter's security.
The four moves
THE TRUNK
Post never delivered
A European postmaster's trunk of 300-year-old undelivered mail still holds 577 letter packets that have never been opened.
Look inside
THE FOUR MOVES
One sheet,
four moves.
Most locked letters combine a few simple moves. Researchers read these traces on old letters to work out how they were closed.
Leave a blank margin
Some locks are cut from the blank edge of the sheet, so the writing needs room around it.
Fold it small
The Brienne packets studied by scanning each measure about 50 by 80 millimetres.
Tuck, slit or thread
A tucked flap, a slit or a paper strip pushed through the folds holds the packet shut.
Seal it
Sealing wax, a wafer or another adhesive finishes the lock and shows if it has been opened.
TWO NUMBERS WORTH KNOWING
Thousands of letters,
dozens of locks.
One figure on how much has been studied,
one on how elaborate a single lock could be.
250,000
Historical letters studied
The research team studied 250,000 historical letters to build the first chart of letterlocking categories, numbered 1 to 64 and each given a security score.
THE STUDY · NATURE COMMUNICATIONS
30+ steps
Steps in one of the most secure locks
The spiral lock needs more than 30 steps. A narrow lock is cut from the blank margin and sewn through the folded letter like a needle.
THE SPIRAL LOCK · NPR
The researchers say the more secure locks made tampering evident to the person receiving the letter, and that how a letter was locked could signal the sender's class and education.
INSIDE THE TRUNK
What does the
trunk hold?
Pick a group of items in the Brienne Collection, the postmaster's trunk of undelivered post. The bar shows how large each group is.
Figures from the 2021 Nature Communications paper on virtual unfolding of sealed letters, Table 1.
READING WITHOUT OPENING
Opened by X-ray,
not by knife.
Archives used to cut sealed letters open on request, which destroyed the folds. In 2021 a team scanned unopened packets with X-ray machines designed for dentistry and unfolded them virtually.
The method read an unopened letter dated 31 July 1697 for the first time, asking a cousin in The Hague for a certified copy of a death notice. The paper's folds and seal stayed intact.
In conversation
Questions people ask
What is letterlocking?
The historical practice of folding and securing a flat sheet of paper so that it becomes its own envelope. MIT Libraries say it was an everyday activity for centuries, across cultures, borders and social classes.
When did people stop letterlocking?
The 2021 Nature Communications paper says most letters were sent this way before mass-produced envelopes spread in the 1830s.
What is the Brienne Collection?
A European postmaster's trunk of 300-year-old undelivered post, now at Sound and Vision in The Hague. It holds 3,148 catalogued items, including 577 letter packets that have never been opened.
How can a sealed letter be read without opening it?
Researchers scanned packets with X-ray microtomography, using machines designed for dentistry, then used an algorithm to separate the paper layers and flatten them on screen.
What is a spiral lock?
One of the most secure methods. NPR reports it takes more than 30 steps: a narrow strip is cut from the margin and sewn through the folded letter. Mary, Queen of Scots used one on her last letter in 1587.
Sources
Each figure on this page comes from the sources below.
- Nature Communications (2021): Unlocking history through automated virtual unfolding of sealed documents imaged by X-ray microtomography
- MIT News: Researchers virtually open and read sealed historic letters
- King's College London: How letters were kept secure for centuries before the invention of the modern envelope
- NPR: Experts crack the secret to the last letter of Mary, Queen of Scots