Many words contain a trigram comprising three consecutive letters of the alphabet. The most commonly occurring are DEF (DEFine, DEFeat, DEFt, etc.) and STU (STUck, STUb, paSTUre, etc.).
"Alphabetic Trigrams and Tetragrams,"
Word Ways: Vol. 32
, Article 7.
Available at: http://digitalcommons.butler.edu/wordways/vol32/iss2/7