The 2-letter word MO can be extended by one letter at a time up to the 9-letter MODERNEST, with all combinations being allowable words in international Scrabble: MO, MOD, MODE, MODER, MODERN, MODERNE, MODERNES, MODERNEST. Seven of these are found in the Official Scrabble Players Dictionary, 4th Edition, 2006. The exception is MODER, a layer of humus intermediate between mor and mull (Chambers Dictionary, 1998).
Word Ways: Vol. 42
, Article 6.
Available at: http://digitalcommons.butler.edu/wordways/vol42/iss3/6