When the number of books in the Great Library of Bridgeport reached a critical number that was not manageable with a handful of lists anymore, the librarians had a tedious job to do: get out all the books, sort them, make an index, and put the books back in. They had one problem though: the dangerous, secret, forbidden, or otherwise disconcerting books and documents. They sat together for three days and created a complicated system of index books and wooden rulers with letters, numbers and punched out holes on them, that enables them to find any book in a short time, but only if you know how the whole system works. As input to the system the book title, the category, the author's name or the date of the book can be used. Depending on the type of the input, certain combinations of letters and numbers lead to other combinations and so on, until the result is a row of numbers: room number, cupboard or table number, row number and finally the number of the book in that row. The dangerous books got unremarkable wrappers and have been placed in a way that they do not get found by accident. At one time an unhappy library assistant had stolen one of the wooden rulers. That caused quite some disturbance until the ruler got returned, and the librarians made several copies of the whole system just to make sure the library is accessible at any time.
The index/code navigation to find a book is based on simple math, veiled by number to letter transitions, shifts sidewards and replacements only possible to do for people knowing the special librarian vocabulary.
SC2020: Write about a secret code or cypher in your world: who uses it, and for what purpose?