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A tattle tag is a device placed in a library item which causes an alarm to go off if it is removed from the building. This is done to deter theft.
Tattle tags are traditionally simple magnets that are detected by a security gate at the library's exit. 3M's Tattle-Tape™ brand of magnetic strips are a popular brand. They may be put in the spine of books, stuck to a blank page or magazines, or placed elsewhere for other media such as books on tape. Although some rare books may be deemed too valuable to potentially damage with tattle tags, they are often not in the open stacks anyway and must be signed out for access.
The tag magnets may be desensitized when the items are checked out, or handed over a security gate and remain charged. The latter method causes many problems. The patron checking out the items may not ready to leave the library, or if any patron wishes to leave the building with an item already checked out, complications and vulnerabilities may arise. Also, if the circulation desk is not immediately adjacent to the library exit, there is no way for the gate guard to reliably verify if the library items being passed through the security are in fact checked out. A stamped due date, for example, may be forged or exist on an item that was returned early if the due date was not crossed out.
Even if checked out items are properly demagnetized, materials from other libraries (or video stores, etc.) may set of a library's alarm. Large magnetic security stickers can be easy to spot, making them vulnerable to removal by potential library thieves. Signals from tags may also be suppressed by holding metallic items such as scissors over them, or lowering or raising them to a height not scanned by the metal detector. Thieves may also attempt to steal a book by hiding it in a pile of checked out books or exploit the vulnerabilities listed above by using other social engineering tactics to fool loss control personnel.
Theft or attempted theft of library materials is a crime. If you are ever near a library exit and the security alarm goes off, please be considerate to library staff and re-enter the gate. Likewise, if you have accidentally forgot to check out an item you are carrying, being polite about the situation is far more likely to avoid getting yourself arrested than is dishonest behavior.