The coding method used was first published by Howard Savage in the June 1929 issue of The Sphinx, Vol. 28 No. 4, p. 136. Savage's code was employed in a book test using a booklet of poetry with about twenty pages. The page number was transmitted, so that the medium could refer to a duplicate booklet and identify the poem selected. The application of his code to playing cards seems to have been made by William McCaffrey (see John N. Hilliard's Greater Magic, 1938, p. 566). See the May 1942 issue of The Sphinx (Vol. 41 No. 3) for J. G. Thompson and Charles Nyquist's survey of telephone test methods, “Thanks to A. G. Bell”, p.49.

See also Telephone Codes.