23B Page 79
23B THE CHOICE OF RUNS
(a) ΔD Statistics.
The choice is based on the statistics of ΔD letter counts for messages already set (22H). Some ΔD characteristics are permanent and common to all links, others are peculiar to a particular link, or to one end of a link, or to particular messages.
|In almost all messages /, 5, U, 8 are common;|
|B is rare;|
|In Type A (stroky) messages / is very common;|
|In Type B (language) messages 3, J, F, G, are common.|
Originally the Berlin ends of Western links were type B, the outer ends of most Western links and both ends of Eastern links were type A. Later the situation became confused and so did the notation A, B, C... [of 22 G(c)]
(b) Practical Runs.
For rapid setting it is insufficient to remember the frequencies of the 32 individual letters: it is necessary to know explicitly what are the best runs for direct application; e.g. to know that 1 + 2 = . is the best run involving only two wheels. This is of course deducible from the 32 letter count, largely because 1+2/ is satisfied by all the letters /, 5, U, 8, 3, J, and not by B. Succint rules are given in "trees".
(c) A simple tree.
(i) Runs bracketed are to be tried, in order, till one of them gives a