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, Dai Vernon is reported to have claimed that he created the idea c. 1925. Following this, Stephen Minch, echoing Vernon's opinion, states that Ralph Hull neglected to credit Vernon when he published his application, an oversight Hull made with other Vernon material.