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Interlock is a way of positioning cards such that the bottom card of the upper half of the deck is under the top card of the lower half of the deck. This condition, among other things, allows you to ditch, change or vanish cards.
Nelson Hahne and Joe Berg described a simple use for the “interlock” configuration in Here's Magic, 1930, p. 17-18, in a trick called “A Mysterious Change”.
The principle later appeared in R. W. Hull's The Elusive Joker, n.d. (c. 1931), p. 2-3. Hull makes no claim of originality. In fact, on p. 11 he states, “This shift of second card is made possible by the arranging of the two packets. It is really not a new move, but has been used in different variations for years…” In The Vernon Chronicles, Vol. 2, 1988, p. 149, author Stephen Minch tells of Dai Vernon's claim to have created the move circa 1925. Minch further suggests that Ralph Hull neglected to credit Vernon when he published the move, an oversight Hull made with other Vernon material.