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Concealment devices or diversion safes are used to hide things for the purpose of secrecy or security. They are made from an ordinary object such as a book, a candle, a can, or something as small as a coin. The idea is that such an inconspicuous object would not be expected to contain anything of worth.

Examples in espionage include dead drop spikes for transferring items to other people, and hollowed-out coins or teeth for concealing suicide pills. Examples in smuggling include suitcases with false bottoms for hiding contraband.

During World War II, a British organization called MI9 was responsible for creating many concealment devices for "escape aids" to assist prisoners of war to escape.

Main article: Concealing something in a book

Books are possibly the most common concealment devices in common usage. They are easily made and can contain quite large objects. They are also very difficult for outsiders to spot but easy to recognize for those that are looking for a specific book on a shelf.

Book safes are often seen in film and television series, with the Bible being the most commonly-used one.^{[citation needed]}

A new type, the hollow candle looks like a large scented candle but is largely hollow. The bottom comes off and rolled papers and small articles of jewelry can be placed inside.

Also a new type, mock spray cans of various household chemicals can be purchased. These have a bottom that comes off and hold articles of the same type as the candles mentioned above.

A hollow container, fashioned to look like an Eisenhower dollar, is still used today to hide and send messages or film without being detected. Because it resembles ordinary pocket change, it is virtually undetectable as a concealment device. If a hollow coin is suspected, it sometimes can be easily confirmed by weighing against a normal coin on a simple balance. However, more sophisticated hollow coins have had their weight adjusted to match an unaltered coin by including a ring of a dense metal such as lead inside the coin.

Such hollow coins were created from two ordinary coins, by milling out one face and the interior of both coins (to create a cavity), and the edges of one (so it could slide into the other). The half coin with intact edges would also have a pin-prick size hole drilled through its face, so the device could be opened by inserting a pin. A scratch may be added to help line up the faces while closing it—although it is very difficult to detect a slight misalignment by casual inspection. A device of this nature was famously discovered by a paper boy in the "Hollow Nickel Case".

A device whereby a safe-looking safe is left open but has a hidden compartment (e.g., in the door) where small valuable articles can be hidden.

You can hide thin objects such as a papers/money in or behind the frame of a painting.

• Concealment
• Rudolph Abel