Check back here soon for a comprehensive glossary of more than 700 letterlocking terms. Can't wait until then? Here are a few terms as a preview! Many of the pictures link to instructional videos. Go ahead and click on them. 

Cite as: Jana Dambrogio, Daniel Starza Smith, et al. 2016. Dictionary of Letterlocking (DoLL). Last updated: 1 November 2016. Date accessed: [Date]. Abbreviated on this page: (DoLL 2016)


Address: The particulars of the location where a person or organization can be contacted by post. Before the invention of the envelope, the address was typically inscribed on the outside of the folded letter packet itself, or on a wrapper. Addresses on locked letters can therefore often be found on the reverse of the letter after it has been unfolded. Address details may consist of a street name, the name of a town or district, or a local landmark. Before the eighteenth century, buildings did not typically have street numbers, and postal codes were not introduced until the twentieth century. (OED, BealDoLL 2016)



Adhesive: “Substances, in the form of a liquid, paste, powder, or dry film; used for sticking or adhering one surface to another. Substances capable of holding materials together by a surface attachment.” Language of Bindings

Adhesives (e.g. seals and wafers) are used in letterlocking as a means of securing a letter shut. They are usually applied to two overlapping sections of substrate, affixing them together. Common adhesives include beeswax, shellac, and other resins; plant starch; gelatin; and pressure-sensitive or heat-sensitive synthetic materials. Clay and mud are used similarly in earlier examples of document security. Adhesives constitute one of the most important, and frequently the most obvious, structural and security devices in letterlocking, since their adhesion must be intentionally broken if the letter is to be opened. They interact closely with folds, stabs, and other kinds of intentional damage in the letterlocking process. (DoLL 2016)


Bifolium, -a

Bifolium: (plural: bifolia) A pair of conjugate leaves (i.e. four pages) of paper or vellum, of any size. A sheet folded in two. (Beal; DoLL 2016)



"The profession devoted to the preservation of cultural property for the future. Conservation activities include examination, documentation, treatment, and preventive care, supported by research and education." AIC.



"A professional whose primary occupation is the practice of conservation and who, through specialized education, knowledge, training, and experience, formulates and implements all the activities of conservation in accordance with an ethical code such as the AIC Code of Ethics and Guidelines for Practice." AIC




  1. A wrapper: “That in which anything is enveloped” (OED, n.1). In DoLL, wrappers are distinguished from envelopes because envelopes are purchased pre-made (see 2 and 3 below), whereas wrappers are improvised from available materials by the letter-writer or secretary.

  2. In epistolary usage: a flat paper container for letters or documents with a flap usually bearing applied adhesive (e.g. gummed or pressure-sensitive). An envelope encloses and protects a letter or card, carries it to its destination, and prevents it from being read by unintended recipients. It bears the postage and address, and does not serve as a substrate for content.
        “The cover of a letter; now a small sheet of paper folded and gummed to serve as a cover for a letter” (OED, n.2)
        a sheet of paper folded and gummed to serve as a separate cover enclosing a letter” (Beal, 139).

3.) In modern usage: a commercially viable factory-made version of definition 2. Gummed envelopes were invented in the 18th century and began to be mass-produced in the early 19th century. The widespread availability of this technology reduced the practice of letterlocking. 
    The use of envelopes only prevailed gradually, especially since it added to the weight and therefore to the cost of postage. Until the Uniform Penny Postage of 1840 standardized the basic postal charge, many letters continued to be sent with the address written simply on the exposed panel of the last folded leaf.” (Beal, 139)

In letterlocking: by definition, locked letters do not use envelopes. Locked letters are folded and secured to function as their own envelopes, but do not become envelopes. (DoLL 2016)



“Chiefly literary. Of or relating to letters or letter-writing.” OED


Floss (textile)

Floss: In letterlocking, a soft thread (usually silk or cotton) used as a “dry” mechanical lock to fasten a letter shut. In some letterlocking formats (e.g. Elizabethan pleated letter), floss is wrapped around the open end of the letter several times before being sealed with adhesive (most commonly sealing wax) applied warm over the floss to fix it in place. Floss can also be used to sew letters shut (e.g. Letter sent from Simeon Fox to Thomas Wilson, 1602, The National Archives, Kew, UK, TNA SP 101/81 ff 344-345).  See “dry” mechanical lock, lock, letterlocking formats. (DoLL 2016)

Link to:




Accordion fold

Corner fold


Gate fold and closed gate fold

Roll fold


Independent folding

Tandem folding


Mountain fold: A fold that is concave upwards when the substrate is laid flat.  /\ (DoLL, OrigamiUSA)

Valley fold: A fold that is concave downwards when the substrate is laid flat.  \/


Diagonal fold

Triangle fold


Folds, how to offer instruction on which way to fold paper.

Folding long-edge-to-long-edge

Folding short-edge-to-short-edge


(also foredge, fore edge) In a codex, the exposed surface of the text pages directly opposite the spine. In a bifolium manuscript, the fore-edge is the edge opposite the fold. (DoLL 2016).



Iron Gall Ink

Gall Invisible Ink


Artichoke Invisible Ink

Alum Invisible Ink


Intentional damage

In letterlocking, loss or detriment purposefully made to a substrate to perform:

  • the act of letterlocking (e.g., cutting off corners to create locks to secure the letter shut, folds to reduce the sheet to a small, portable packet, and slits to use as conduit with which to insert a mechanical lock to secure the letter shut)

  • The act of opening a letter (tearing open a security tab or cutting gloss, tearing around a seal or cracking it open)

  • The act of reformatting the letter to fit into a storage container (cutting and folding an edge of a letter to fit into a standardized letter book, folder, or box).

In the study of letterlocking, the damage may help identify an individual’s particular letterlocking habits. (DoLL 2016)


Letter: A written text on paper, parchment, etc., and related senses.” OED




1. The act of folding and securing an epistolary writing substrate (such as papyrus, parchment, or paper) to function as its own envelope or sending device.

As distinct from the use of a wrapper or gummed envelope, and from origami, the art of paper folding.

2. A sub-category of a 10,000-year information security tradition, pertaining to epistolary materials.  

3. The discipline which studies the materially engineered security and privacy of letters, both as a technology and a historically evolving tradition. 

(DoLL 2016)


Letterlocking Format Categories for opened and unlocked letters

Visit the Categories page here. 


Manuscript : “ Of a book, document, etc.: written by hand, not printed.” OED


Margin folds


Micro-tomography, time delay integration, computed

Microtomography, time delay integration, computed: Courtesy of The Apocalpto Project. A video showing the layers of a high contrast TDI CT (Time Delay Integration, computed micro-tomography) test scan of a bundle of 20 locked letters. Using historic letterlocking formats, iron gall ink, quill, and sealing wax , the #SignedSealedUndelivered team made up the batch of new letters for testing, writing secret messages inside, folding them using historic letterlocking formats, and sealing them with wax. The Unlocking History team and their collaborators are developing the technology virtually reveal the contents and folding patterns to be revealed without tearing open the letters



Panel: In letterlocking, a flat two-dimensional unit between folds of an epistolary writing substrate.



Philately: "(a term coined in the 1860s) denotes the pursuit of postage stamp collecting and also the study of all physical characteristics of stamps, as well as related aspects of postal history and postal stationery, including postal marks, hand-stamps, labels, envelopes, postcards, etc.” Beal


Reflective Transformation Imaging, RTI

“A computational photographic method that captures a subject’s surface shape and color and enables the interactive re-lighting of the subject from any direction. RTI also permits the mathematical enhancement of the subject’s surface shape and color attributes. The enhancement functions of RTI reveal surface information that is not disclosed under direct empirical examination of the physical object.” See examples of RTI used o letters from the Wunsch Conservation Laboratory's Historic Letterlocking Manuscript Collection, MIT Libraries, in the DoLL seal entry image gallery.




1.) An instance or place where the opening of a letter has been prevented/inhibited

2.) The adhesive as applied in meaning 1, e.g. wax.

“A piece of wax or some other plastic or adhesive substance (commonly, and still frequently, one bearing the impression of a signet ... fixed on a folded letter or document, or on a closed door or receptacle of any kind, in such a way that an opening cannot be effected without breaking it.” OED, “seal”, n2, 2a

3.) The device that impressed into the adhesive (meaning 2); can be referred to as seal-matrix, seal-die, seal-ring, signet-ring, seal-stamp, and stamp.

A device (e.g. a heraldic or emblematic design, a letter, word, or sentence) impressed on a piece of wax or other plastic material adhering or attached by cords or parchment slips to a document as evidence of authenticity or attestation.” OED, “seal”, n2. 1a

4.) The low-relief design cut into the face of this device (meaning 3).

    “A device or inscription engraved on a seal.” OED, “seal” n2, 3d.

5.) The impression left behind on the adhesive (meaning 2) by the engraving (meaning 4) on the device (meaning 3), whether directly (exposed seal) or on paper laid over that adhesive (papered seal). Often incorporating an authentication and/or decorative device (such as a family crest).

6.) Metaphorical meanings derived from the above (e.g. seal of approval, my lips are sealed).

(DoLL 2016)

Sealing Wax

Images of the Unlocking History team's Brien Beidler teaching the rest of us how to make sealing wax from historic recipes. Have an old recipe? Send it to us here.

1) any of a class of pliable substances of animal, plant, mineral, or synthetic origin that differ from fats in being less greasy, harder, and more brittle and in containing principally compounds of high molecular weight (e.g., fatty acids, alcohols, and saturated hydrocarbons). Waxes share certain characteristic physical properties. Many of them melt at moderate temperatures (i.e., between about 35° and 100° C, or 95° and 212° F) and form hard films that can be polished to a high gloss, making them ideal for use in a wide array of polishes. They do share some of the same properties as fats. Waxes and fats, for example, are soluble in the same solvents and both leave grease spots on paper.’ (Encyclopaedia Britannica)

2) In letterlocking, sealing wax can be used as a generic term to describe the adhesive melted and applied to secure a letter shut. Most ‘sealing wax’ used from the early modern period onwards is actually a resin-based compound.

“In early use, beeswax (or a mixture of this with other substances) as employed to receive the impression of a seal; in later use, a compound, chiefly consisting of lac, serving the same purpose” (OED, ‘wax’, 4a)

In early use, beeswax or a composition containing this, in later use a composition consisting of shellac, rosin, and turpentine, prepared for the purpose of receiving the impression of seals. (OED, ‘sealing wax’, 1

3) ‘In figurative and similative uses, referring to the easy fusibility of wax, its softness and readiness to receive impressions, its adhesiveness’ (OED, n1.3a).


Secret Writing Techniques

Secret writing techniques include methods devised to communicate discretely. Some of the videos in the above gallery showcase our collaborative work with Dr. Nadine Akkerman. Akkerman researches 17th-century female spies and their secret writing techniques. Other videos show coded messages we've found in locked letters.



“The articles sold by a stationer; writing and office materials. In later use freq.: spec. writing paper and envelopes. A stationer is a person or shop selling paper, pens, and other writing and office materials.” OED  


Substate, Writing

Substrate, Writing: "An underlying substance or layer." OED



“A watermark is the device or design impressed into a sheet of paper as a result of the manufacturing process. In early papermaking, wires in the tray or mould were twisted into various shapes, the paper produced being thinner where the fibres touched the wires, leaving a design which was visible when the paper was held up to the light. Although certain types of design (such as pillars or posts, bunches of grapes, Strasbourg lilies, and pots and urns) are so common as to be virtually indistinguishable, watermarks were originally used as trade marks, and many found in early modern paper can denote, or be associated with, particular manufacturers, as well as with particular places and dates. Certain watermarks, such as a foolscap or post horn, were also used to distinguish particular paper sizes. The range of watermark designs in hand-made paper is vast, ranging from simple lettering or geometrical shapes, through figures such as animals, birds, flowers, insects, horns, and croziers, to elaborate beehives, scrollwork, and coats of arms.” Beal


Wax gun

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A low temperature hot glue gun used for melting and applying sticks of adhesive labeled as glue gun waxes and faux sealing wax. (DoLL 2016)



"Precursors to envelopes. Paper wrapped and sealed around a folded letter. A wrapper: “That in which anything is enveloped” (OED, n.1).

In DoLL, Wrappers are distinguished from envelopes because envelopes are purchased pre-made, whereas wrappers are improvised from available materials by the letter-writer or secretary Both wrappers and envelopes are types of containers. (DoLL 2016)