Singularly Imperfect Tautonyms (SITS for short) are what I call words like NIGHTLIGHT in which a single letter in the second half of the word differs from the letters in the corresponding position in the first half of the word. Thus a single letter prevents NIGHTLIGHT from being a tautonym (NIGHTNIGHT). The SIT MENDMENT is prevented by a single letter from being a tautonymic sequence, rather than a tautonym, MENDMEND not being a word. For the purpose of this article, I do not distinguish between these two types of SITS. As a genre, SITS are only to be found amongst words with an even number of letters. Word sources can be found at the end.
"Singularly Imperfect Tautonyms,"
Word Ways: Vol. 36
, Article 3.
Available at: https://digitalcommons.butler.edu/wordways/vol36/iss3/3