If adding two numbers to get 26 letters, you only need the values 1 through 13. Without a zero, however, you would need to be able to sum negative numbers as well, since 1 + 1 = 2 (B) misses the letter A. So, when converting letters to numbers, for A through M we count the letters from the beginning of the alphabet; for the letters N through Z we count from the end of the alphabet for negative numbers. So:

A = 1, J = 10, K = 11, (ZYXWVUTSR are 9 letters), Q = -10

    8    4  11   1  4
   11 10 -10 10  1
   ---- ----- ---- ---- ---- ("---" = BAR)
   19 14   1  11   5
     S   N   A   K   E

 

(I think the explanation is courtesy of +Robert Simpson. Thanks !)

 

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