By the time we reach six-letter words, conditions for superior ladders are much improved: nearly one-half of six-letter words are heterograms (all letters are different), the fraction of isolanos (words which have no neighbours) decreases to less than one-tenth, there is a reasonable number of onalosis (words which have neighbours for each letter change), and each word has on average almost six neighbours (one for each letter). In this article, we therefore only consider ladders in which the terminal words are heterograms, with corresponding letters different, and with letters replaced in order. Even so, there are about fifty thousand of them.
"Six-Letter Connoisseur's Ladders,"
Word Ways: Vol. 34
, Article 15.
Available at: http://digitalcommons.butler.edu/wordways/vol34/iss1/15