all the cards are suddenly presented as being all the same as the initially chosen card. Joe Stuthard's Trilby and Bi-Co Trilby Decks are variations on this deck. In the 1960s and 1970s, Marshall Brodien sold 17 million Svengali decks under the name TV Magic Cards. While the Svengali Deck allows for the easy performance of seemingly complex tricks, the conjurer cannot allow an audience member to examine the deck. The use of a Svengali Deck can also be detected by its characteristic faster riffle and sound. Finally, because the deck is widely sold in joke and novelty shops, many laypeople are familiar with it. Writing pads or books using the same principle have also been created, to force the spectator to choose the content of a seemingly random page. Marked deck A marked deck uses a marking system on the back of the cards to indicate their suit and value, so that a magician can identify a card without seeing its face. The two main methods used for creating a marked deck are reader systems, which hide the suit and value in the back design, or coded systems, where the suit and value is indicated by a code, e.g. a mark might be placed in a certain position to indicate the number of the card, with the color or shape of the mark indicating suit. Many marked decks are produced commercially, while some are created using card marking techniques similar to those used to cheat at gambling, e.g. by adding tiny punctures or dots. Some magicians may even mark a card during a performance, either with a sharpened finger nail or a tool (often a ring known as a "poker ring") made specifically for that purpose. Forcing decks These decks are used to force a spectator to select a particular card, which the magician already knows in advance. Such a feat can be accomplished using a regular deck and sleight of hand but with the aid of a forcing deck the trick is made self-working. One of the simplest forcing decks is the "one-way forcing deck", where all cards are identical (with the possible exception of one or two at the top and bottom of the deck). When using this deck, the magician must be careful to prevent the spectator from seeing the faces of the cards. The above-mentioned Svengali Deck can also be easily used as a force