Repeat, slightly modified, of a previous post that got shunted to the attachment heap. Yes, intimately. They had no relationship to the PDP 10. The first one was a fantastic tour de force by Bob Morris, called "typo". Aside from the file "eign" of the very most common English words, it had no vocabulary. Instead it evaluated the likelihood that any particular word came from a source with the same letter-trigram frequencies as the document as a whole. The words were then printed in increasing order of likelihood. Typos tended to come early in the list. Typo, introduced in v3, was very popular until Steve Johnson wrote "spell", a remarkably short shell script that (efficiently) looks up a document's words in the wordlist of Webster's Collegiate Dictionary, which we had on line. The only "real" coding he did was to write a simple affix-stripping program to make it possible to look up plurals, past tenses, etc. If memory serves, Steve's program is described in Kernighan and Pike. It appeared in v5. Steve's program was good, but the dictionary isn't an ideal source for real text, which abounds in proper names and terms of art. It also has a lot of rare words that don't pull their weight in a spell checker, and some attractive nuisances, especially obscure short words from Scots, botany, etc, which are more likely to arise in everyday text as typos than by intent. Given the basic success of Steve's program, I undertook to make a more useful spelling list, along with more vigorous affix stripping (and a stop list to avert associated traps, e.g. "presenation" = pre+senate+ion"). That has been described in Bentley's "Programming Pearls" and in http://www.cs.dartmouth.edu/~doug/spell.pdf. Morris's program and mine labored under space constraints, so have some pretty ingenious coding tricks. In fact Morris has a patent on the way he counted frequencies of the 26^3 trigrams in 26^3 bytes, even though the counts could exceed 255. I did some heroic (and probabilistic) encoding to squeeze a 30,000 word dictionary into a 64K data space, without severely affecting lookup time. Doug