There are about 7800 different trigrams which occur in words listed in boldface in Webster's Second Edition, making it feasible to compile a dictionary of types. Approximately 80 per cent of these trigrams (those appearing in Webster's Collegiate) were published in the August 1969, November 1969 and November 1970 Word Ways; since that time, Philip Cohen of Aliquippa, Pennsylvania has discovered examples for most of the remaining Websterian trigrams, as well as others in Webster's Third. However, a type of dictionary for tetragrams or longer letter-sequences is a far more laborious undertaking, requiring the assistance of a high-speed digital computer; for example, it is likely that more than half a million different seven-letter sequences exist. Consequently, one must focus attention on long letter-sequences having interesting properties.
Card, Leslie E. and Eckler, A. Ross
Word Ways: Vol. 7
, Article 8.
Available at: https://digitalcommons.butler.edu/wordways/vol7/iss1/8