Superimposed code

A superimposed code such as Zatocoding is a kind of hash code that was popular in marginal punched-card systems.

Many names, some of them trademarked, have been used for marginal punched-card systems: edge-notched cards, slotted cards, E-Z Sort, Zatocards, McBee, McBee Keysort, Flexisort, Velom, Rocket, etc. The center of each card held the relevant information—typically the name and author of a book, research paper, or journal article on a nearby shelf; and a list of subjects and keywords. Some sets of cards contained all the information required by the user on the card itself, handwritten, typewritten, or on microfilm (aperture card). Every card in a stack had the same set of pre-punched holes. The user would find the particular cards relevant to a search by aligning the holes in the set of cards (using a card holder or card tray), inserting one or more knitting-needle-like rods all the way through the stack, so the desired cards (which had been notched or cut open) fell out from the irrelevant cards in the collection (left un-notched), which remain on the needle(s). A user could repeat this selection many times to form a complex Boolean searching query. A card that was relevant to 2 or more subjects would have the slot(s) for each of those subjects cut out, so that card would drop out when either one or the other or both subjects was selected . The "superimposed code" coding systems, such as Zatocoding, saved space by entering several or all subjects in the same field; such a "superimposed code" stores much more information in less space, but at the cost of occasional "false" selections.

Once you have a collection of index cards, one per book, research paper, or journal article in a library, with a list of keywords (subjects) discussed in a particular book written on that book's card, the "obvious way" to code those subjects is to count up the total number of subjects used in the entire collection R, make a row of R holes near the top of every card, and for each subject actually discussed in a particular book, cut a slot from the hole corresponding to that subject in the card corresponding to that book. Naturally, this also requires a separate list of every subject used in the collection that indicates which hole is punched for each subject. Unfortunately, there may be thousands of distinct subjects in the collection, and it is impractical to punch thousands of holes in every card. While it may not seem possible to use less than 1 hole per subject, superimposed code systems can solve this problem.

The Zatocoding system of information retrieval was developed by Calvin Mooers in 1947.

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