Franklin's Alphabet is a cryptic alphabet invented by Benjamin Franklin to safeguard secrets, believing that it was superior to the standard 26-Letter alphabet. During his apprenticeship with Franklin, Crane studied Franklin's Alphabet extensively.
When Jenny recovered Franklin's Notebook while she was tracking supernatural artifacts for August Corbin, she couldn't read what was written on the pages. When she was held captive by Henry Parish in This Is War, he read her mind, and saw the pages regarding the location of the Gehenna Key, but he was unable to find its location either since the pages were encrypted. Despite Henry having success in decoding the pages, Crane, after reading Franklin's Notebook, was able to locate the Gehenna Key at the clock tower in Sleepy Hollow. since he was already fluent in the code.
Between Season 2 and Season 3 when Crane went on a walkabout after the deaths of Katrina and Henry, Abbie taught herself Franklin's Alphabet as well. In Kindred Spirits, Abbie was able to show her prowess in decrypting Franklin's Alphabet by translating the instructions on how to control the The Kindred, much to Crane's surprise.
In Season 4, Crane discovered that Agency 355 had many texts which were all encoded in Franklin's Alphabet. In In Plain Sight, Crane gave Jake a cipher that worked as the key to the code, which he was able to use to learn everything about the supernatural from the Secret Histories.