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In reply to:
codes of the Playfair type
A Playfair cipher (it's not really a code)substitutes letters in pairs from a simple grid. Dorothy Sayers, who was fond of such problems, gave a detailed description of the cipher and how to crack it when she has Lord Peter and Harriet discover and solve one in Have His Carcass.
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Carpal Tunnel
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Carpal Tunnel
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WOW:
What memories you brought back. My parents were both journalists, one a reporter and the other an editor. My mother was a police and courts reporter for the Alexandria, Virginia Gazette, which boasted that it was the oldest continuous daily newspaper in the US. Now defunct, of course, much to my dismay.
I still have my line of type with my name on it that I received on a visit to the press room not quite 50 years ago. And I definitely remember the strange keyboards and the even stranger men who worked at them. They all smoked unfiltered Camels which they lit by touching the tip to the bucket of hot lead which fed the machine its raw materials.
In fact, they still set some type by hand for display ads when I first started hanging around. I too wish I had known now much the cases would be worth, because there were stacks of them, many still containing their fonts, gathering dust in a shed behind the building.
Those were glorious times, and I suspect the world is poorer for the changes that have come to the newspaper world.
TEd
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historical accident of qwerty keyboardsNot an accident at all. They made the qwerty so that the common patterns of keystrokes would be far apart so that the typewriter wouldn't get tangled up as easily. it's a conspiracy, i tell you p.s. why does AEnigma interpret qwerty as rabbit?  It's time for the human race to enter the solar system. --Governor George W. Bush Jr.
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Pooh-Bah
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Pooh-Bah
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>They all smoked unfiltered Camels which they lit by touching the tip to the bucket of hot lead
Yes, those Health & Safety people spoil all the fun these days, don't they? I remember the days when chemistry lessons had just the right number of explosions per lesson to keep everyone awake, they stopped those too.
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old hand
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OP
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historical accident of qwerty keyboards
Not an accident at all. They made the qwerty so that the common patterns of keystrokes would be far apart so that the typewriter wouldn't get tangled up as easily.
True enough, xara, but one presumes that there were a number of ways in which this could have been done, so the particular arrangement we have is, to that extent, a matter of chance. More importantly, however, there was the opportunity to change to a more rational scheme (a simple alphabetic one, if I remember rightly) that we missed because the winner of a touch-typing competition around 120 years ago happened to be a man who used a qwerty keyboard. As a result, qwerty became established (for speed! not for slowness!), and is now ubiquitous (except, as we can see, amongst linotype operators).
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they are the most frequently used letters - as most of the code-breakers below have appreciated Now, hold it here a second, folks. It just occurs to me that I've always blithely accepted statements about "the most frequently used letters" and, very closely related, "the most commonly used words" in the English language. How are these "facts" known?? OK, in a particular context for a certain period of time you can make a broad-sweep judgement. Also it's a lot easier these days to produce accurate mathematical counts for a number of documents, and then extrapolate from there. But but but.. like all statistics, we don't have any absolutes. In many contexts, for instance, articles will be omitted, words abbreviated and slang introduced. Surely this makes a significant difference to word and letter counts. And another thing - language evolves over time. New words and phrases appear, and old ones fade away. Sometimes (as we know) this happens very quickly. Codebreakers would certainly need to take account of such factors, as top secret messages are only going to say what has to be said, in as condensed a form as possible. Context is absolutely crucial, as that will dictate the most frequent phrases/words/abbreviations. I'd therefore like to ask: Who came out with these frequency figures, how were they determined, and when? Nice simple question! 
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Pooh-Bah
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Pooh-Bah
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So would you go with Dvorak then?
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Carpal Tunnel
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No, thanks. I prefer Sibelius myself :)
TEd
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So would you go with Dvorak then?Nah, I'll just go with the flow, Jo!  Zen Fish (Zen bones)(Zen - dry bones..)
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old hand
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... Simon Singh's The Code Book is a brilliant cyphers-for-amateurs work (and there's a large-ish prize at the end, if it hasn't been claimed yet).
1. Letter frequency: Usually, I believe worked out from various corpuses, of a million words or more each. Simon Singh gives various frequency details from different corpuses. In any large enough body of words in English we find a surprisingly similar set of frequencies. Etaoin shrdlu may not be an absolute representation of relative frequencies, but it is a very useful one for a codebreaker (yes I know it's technically called a cypher, but if Simon Singh can call it code, I can too).
2. Letter frequency is not used in any absolute sense when codebreaking (ok, ok, deciphering). See, for instance, the classic story - Sherlock Holmes' "Adventure of the dancing men", to see how letter frequency combines with intuition and good ol' fashioned guesswork, to make decipherment possible. All codebreakers rely upon getting their hands on a reasonably susbtantial portion of text, and for whatever reason, I believe that type of use (telegraphic, legal, medical etc) does not skew the frequency distribution too much.
3. The technique was developed by (who else?) our old friends the Arabs (Saracens?), and the word cypher (for another cross-thread) comes from the Arabic (sifr?). In the tenth or eleventh centuries, the agents of the Caliph and others, determined to crack letter-substitution codes, slowly built up their knowledge of letter frequencies in Arabic. Whether this technique was slowly disseminated through Europe, or re-invented later, I don't know, but it has stood codebreakers in good stead for centuries. As Simon Singh points out - Mary Queen of Scots lost her head because of the efforts of a codebreaker using letter frequency tables to help him decipher her secret messages plotting against Lizzie 1.
cheer
the sunshine warrior
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