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Home » About » Blog » Breaking into the Past: An Instance of Code Cracking in the Tinder Postcard Collection

Claire Danna is the current Joyce Bonk Fellow at the Clements library and a graduate student at the University of Michigan’s School of Information. Their primary work at the Clements has involved scanning the Tinder postcard collection and helping to shape the library’s first Zooniverse project. She has also presented on aspects of the collection and its connection to U.S. postal history in the Clements Library Bookworm webinar series.

The David V. Tinder real photo postcard collection provides us with over 60,000 small windows into the everyday life of Michiganians in the early 20th century. While one might be led to believe that 60,000 postcards would be enough to help create a larger narrative about the life and times of those who collected and sent them, these postcards often become sites of profound mystery. Looking at a single postcard, one may be offered only a tiny glimpse into a particular photographer’s work or sender’s experience. These brief but intriguing bits of ephemera open up questions and opportunities for cultivating stories about the people whose lives may or may not have made it into the history books. 

One intriguing example was this postcard sent from Palo, Ionia County, in August of 1911 to Muskegon. While a vast majority of the postcards immediately capture attention because of their images, this one was particularly striking due to its message entirely written in code.

Front of a postcard showcasing a church; the church has nine windows and a steeple. Along the bottom, in handwritten, it reads: Baptist Church.
The back of the postcard - the encrypted message on the left and the address of the recipient on the right.
This encryption method is known as a pigpen cypher, but has also been called a masonic cypher, Freemason’s cypher, Napoleon cypher, or tic-tac-toe cypher. It is a simple substitution cypher where a particular symbol is used to replace a particular letter or character. The symbol used for the substitution is determined by the position of the letter in a series of grids, as shown in Figure 1. Using the standard pigpen substitution (Figure 1), the letter E would be represented by a box, the letter C would be represented by an “c” shape, and the letter W would be represented by a “w” shape.

Substitution cyphers like this are rudimentary in nature and often very simple to crack. Pigpen cyphers were used in North America in the early 18th century by Freemasons, one example of which can be found on the tombstone of James Leeson in the Trinity Churchyard in New York City. During the American Revolutionary War, more advanced alpha-numeric substitution were utilized.  A pigpen code like the one on this postcard would not be used to hide secret information of great importance and would be more apt as a training method for encoding messages.

Another example of a pigpen cypher is found in Adam Ludewig’s 1885 diary. This daily Excelsior diary kept by a young bookseller’s clerk, also from Michigan, included a handful of encoded words or single letter replacements using a pigpen cipher which you can see in Figure 2. Ludewig used the code to disguise specific individuals’ identities or events throughout the diary. These ‘code names’ were especially convenient in the repetition and abbreviation of recurring individuals. Some of them seem to refer to young women in the clerk’s life, a theme also found on the Tinder postcard.
The postcard is unusual in the Tinder collection in that the entire message is encoded. An initial review revealed that the postcard text did not appear to align with the traditional, popular, or common formulations of pigpen cyphers. Using other decryption methods could reveal whether this unique pigpen cypher utilized a recognizable pattern.

Fortunately, the postcard text includes punctuation and periods to separate individual words. Isolating single letter words, which are almost always going to decode to either “I” or “a,” can help uncover words like “in,” “is,” “it,” “at,” “an,” and “as.” Common three-letter patterns like “the” and “you” can then subsequently be decoded and found in words like “they,” “them,” “there,” “then,” “these,” “your,” “yours,” and so on. This postcard starts off with two four-letter words followed by a colon and a dash. Given that this is a letter format, it was likely that the first four-letter word might be “Dear” followed by the name of the recipient. This guess could be confirmed by using frequency analysis.

Decryption processes for substitution codes often begin by conducting a frequency analysis on the pigpen characters present. Because certain letters appear with much greater frequency in the English alphabet, these frequencies can be put into a table and used to fill in a number of letters in a substitution cypher based on educated guesses. The letter “E” is by far the most frequently used letter in the English alphabet, followed by “T,” “A,” “O,” “I,” “N,” “S,” and “R”.

A frequency analysis on the first three lines of the postcard text reveals that this pattern holds up well, with the top five most common letters being “E,” “R,” “O,” “I,” and “N”. The symbol which appeared most and almost 50% more than its runner-up was “c”. We can then experiment by replacing pigpen “c” with “E.” The result matches our supposition that the first word is “Dear”.  By continuing the frequency analysis, assigning “I” and “a” to the next most common pigpen symbols, the message began to slowly take shape. Patterns such as double letters, like the double “w”s in the word “oewwoc”,  or punctuation clues like question marks can help guide the decryption process.

By utilizing all of these techniques, the message was ultimately unveiled. It reads:

“Dear Leon: – I rec’d your everwelcome cards tonight and hope you are having a fine time. Yes, I will be a sweet little girl until you return and maybe after. Ha! Ha! When are you coming home? I want to see you so bad. Boo! Hoo! But such is life. Have you seen any snakes lately? “Fond recollections.” Bye Bye Myrtie”

Figure 1: This diary was a generous donation to the Clements from Angela Del Vecchio in 2018.
Figure 3 This diary was a generous donation to the Clements from Angela Del Vecchio in 2018, titled Wednesday 5, 1885.
Figure 3: This diary was a generous donation to the Clements from Angela Del Vecchio in 2018, titled Monday 28, 1885.
Substitution or translation errors by the writer can complicate the decryption process. For example, in the word “everwelcome,” we see that the sender accidentally left off a dot in the pigpen character.  Unless corrected based on context or against other words in the cypher, “everwelcome” would read “euerwelcome.” Common word patterns like those listed above allow errors like this to be identified more easily.

By mapping the decoded letters used in the substitution back into the pigpen grid pattern, we can see that the correspondents created a clever variation of the cypher by alternating between different grids when assigning letter placements, rather than filling in one grid before moving to another when the spaces were filled. This is perhaps the most satisfying piece of decoding a message like this – being able to see order where once there seemed to be disorder.

Pigpen grid pattern with different variations of letters and dots.

With the text of the message made clear, what opportunities are we now afforded? We can answer a few questions or posit theories. We may also uncover new puzzles that help us recognize the limits of our knowledge based on current evidence. Who are Myrtie and Leon? One might speculate that they are young lovers, friends, or even relatives. What is their inside joke about snakes, as their “fond recollections” in quotations suggests? And while the note has a certain lightness and brevity about it, Myrtle also reveals some acceptance of the hardships of the time in her brief interlude “But such is life.” The everyday nature of the Tinder collection postcards, even when encrypted, has a powerful relatability, so recognizable to every person who has only had a small glimpse into the life of someone passing by them or talking next to them on the phone. The mystery inherent in this message might feel particularly compelling as we reflect on the messages we sent and the confusion, loneliness, and longing we experienced as the result of a global pandemic.

With curiosity, context, and creativity, we might be able to start to tell a story. In the case of Leon and Myrtie, we see the letter is sent to Muskegon and has a note at the bottom that reads “To Camp Grounds.” In Muskegon in the summer of 1911, the Boy Scouts of America held their first official summer camp at the Owasippe campgrounds. Perhaps Leon was a young teenage boy scout. Perhaps he earned a badge for code making and breaking. Maybe he taught his sweetheart the code and told her to write him while he was gone for the summer. And maybe none of these things are true. Maybe there will be another encoded postcard found amidst the Muskegon cards that takes this story in a new direction, or perhaps we will be left to wonder and chase down different curiosities.

The combination of human communication, photography, various places, times, and parties within the boundaries of 3×5 inch pieces of cardstock like this one makes these postcards compelling sites for exploration, research, and storytelling. The Clements Library invites you to come explore them and find your own mysteries or take part in our new Zooniverse project if you are interested in seeing more of what these postcards have to offer.