Three letter rule poets such as John Milton alternated between short and long forms for function words, depending on whether they occurred on or off the meter. For instance: While many function words have more than two letters ("and", "she", "were", "therefore", etc.), the exceptions to the rule are rather two-letter content words. Only a few of these occur commonly in most texts: the words "go" (which also has a functional usage in the idiom "going to do something"), "ox" and, especially in American texts, "ax". English grammar is relatively flexible about converting words of one class to another, allowing verbal uses such
Square-free word first difference of the Thue–Morse sequence. That is, from the Thue–Morse sequence one forms a new sequence in which each term is the difference of two consecutive terms of the Thue–Morse sequence. The resulting square-free word is Another example found by John Leech is defined recursively over the alphabet {"a, b, c"}. Let formula_1 be any word starting with the letter "a". Define the words formula_2 recursively as follows: the word formula_3 is obtained from formula_4 by replacing each "a" in formula_4 with , each "b" with , and each "c" with . It is possible to check that the