Cryptographic History of Work on the German Naval Enigma
B Explanation of the Method.
3. Suppose we have two messages encyphered with machine starting positions
[or set up's] ABG and ABL respectively and that there is no turnover in going
from G to L. Then since L is 5 letters ahead of G in the alphabet after five
letters of the first message have been encyphered we shall reach the position
ABL which is the start for the 2nd message. 
The 6th, 7th, 8th ..... letters of the first message will be encyphered at
the same machine positions as the 1st, 2nd, 3rd ..... letters of the second
message. Writing out the clear texts of the message so that the 6th, 7th,
8th ..... letters of the first message are written immediately above the 1st,
2nd, 3rd ..... letters of the second we get
4. Now we will write out the encypherments of the messages - notice (1) the fundamental point that since letters written in the same column are encyphered at the same machine position, repeats in the clear text become repeats in the cypher text (2) that since both set ups are encyphered at the same machine position (i.e. at the grund for the day) repeats in the set up become repeats in the message trigram (Verfahren kenngruppe).
5. Messages written out in this way are said to be "in depth" with each other. In this example they are "at distance 5" and since it is the distance corresponding to the differences in their indicators it is a "true distance". Only if the distance is "true" do we get the same repeats in the cypher text as in the clear text; at all other distances we get merely random repeats since letters writer in the same columns are not then encyphered at the same machine position.
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