The 1944 Bletchley Park Cryptographic Dictionary

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SLIDE, n :

1. Movement of two alphabets or numerical series written or printed with equal spacing on two rods or other suitable mediums, against each other, or of one alphabet or series against itself, giving for (each different position a different substitution alphabet, etc. (For convenience one of the alphabets is normally repeated.).

2. Instrument consisting of one rod capable of being moved along a groove in another rod, both being divided into equal spaces for the reception of alphabets.

3. Amount which one alphabet (or other series) is slid against another or against itself, measured by the number of spaces a particular letter has moved across from a fixed starting-point, for enciphering a particular letter; normally determined by a letter or figure of the key.

4. (in Tunny). Incorrect relative position of part of a cipher text, occasioned by one or more missing or extra letters, and tending to add to the normal difficulties of chi-setting and/or deciphering.

5. (in Tunny). Similarity, specially of a chi-wheel pattern to itself, usually about four places farther on, giving similarities in the counts obtained from different settings.

SLIDE :

1. To move one alphabet or series of numbers against another or against itself, especially alphabets, etc. written for convenience on rods or a cipher-slide.

2. To compare two different frequency distributions or the like in different successive horizontal positions with a view to fitting one to the other.

SLIDE CODE :

A three letter code which is varied from time to time by sliding the code groups against the clear equivalents.

SLIDE-RULE :

1. A cipher-slide.

2. A log-scale.

SLIDING ALPHABET :

An alphabet which is slid against another or against itself in the process of enciphering or deciphering.

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