If letters are assigned values according to their numerical positions in the alphabet (A=1, B=2, etc.), an eight-letter numerical tautonym is a word in which the sum of the values of the first four letters is equal to the sum of the values of the last four letters. For example, B, A, C and K have the numerical values 2, 1, 3 and 11, which sumes to 17, and A, C, H and E have the numerical values 1, 3, 8 and 15, which also sums to 17; thus BACKACHE is a numerical tautonym.
Card, Leslie E.
"Eight-Letter Numerical Tautonyms,"
Word Ways: Vol. 4
, Article 19.
Available at: http://digitalcommons.butler.edu/wordways/vol4/iss3/19