Peekaboo Card

(also called optical coincidence, Batten, or Cordonnier card), a transilluminatable rectangular card containing data for information retrieval. These cards are cut from thin (0.18 mm) cardboard or heavy paper in 148 × 210 mm, 210 × 297 mm, and 297 × 420 mm sizes. Ordinary punch cards are also used on occasion. Holes are punched on a coordinate grid in the field of the card to designate the address or identification number of the documents containing the specific retrieval key word used in the search. The maximum number of holes that can be punched in one card is 3,500, 7,000, or 22,500, depending on its size.

If several cards are held together and viewed under transillumination, the coincidence of common holes can be seen. This feature is the effect for which the cards have been named. For example, let one peekaboo card contain the keyword “steel,” another the word “machining,” a third “drilling,” and so on. The card with keyword “steel” indexes the numbers of all documents having abstracts that contain the word “steel.” A user who views this card together with the second card will be able to read the numbers for documents containing both the word “steel” and the word “machining” from the coincident openings. The search process can be continued until the only transmitted light identifies the document or item uniquely described by the chosen set of keywords.

Peekaboo cards were first developed in the USA in 1915. They are used in various information retrieval systems under many different names.