The principle was first marketed by Bob Hummer in his “Politicians Puzzle”, in early 1952 (see The Linking Ring, Vol. 32 No. 2, Apr. 1952, rear cover; also see Bob Hummer's Collected Secrets, p. 33). A related item was his “Mom and Pop Puzzle”, which was bundled with “Politicians Puzzle” the following month (The Linking Ring, Vol. 32 No. 3, May 1952, inside front cover; and p. 34 of Bob Hummer's Collected Secrets). Another early application of this parity principle is Mel Stover's “The Immovable Object” in The New Phoenix, No. 340, Jan 10, 1957, p. 172. Martin Gardner's influential “3-by-3 Matrix” application appeared in his Scientific American column, August 1960, and then was reprinted in Hugard's Magic Monthly, Vol. 19 Nos. 1 and 2, Sept. 1961, p. 17. This became the mold for most versions that followed. Hal Newton's “Voice from Another World” was the first of the record-tape tricks using the principle to be marketed. See Fulves's The Fine Print, No. 10, p. 355, and Martin Gardner Presents, p. 149, for further details.