For many years word buffs at these laboratories have made a game out of finding words which contain strange digrams (pairs of letters side-by-side). The problem, as originally posed by M.D. McIlroy, was to complete a table with 26 rows and 26 columns, labeled by the letters A, B, ... , Z. The space where row L crossed column L' was to be filled by a word containing the digram LL'.
Gilbert, E. N.
"The Digrams of Webster's Unabridged Dictionary,"
Word Ways: Vol. 2
, Article 8.
Available at: https://digitalcommons.butler.edu/wordways/vol2/iss4/8