In the November 2004 Kickshaws, Dave Morice pointed out that the first two letters of HOW add to the third (8+15=23), and that the first two letters of to the third (23-25=8). This idea can be extended to longer words; any two consecutive letters in a word must add (or subtract) to the next letter, such as CADE, 3+1=4 followed by 1+4=5. To allow for sums greater than 26, use the circular alphabet: A=1, B=2, ... Z=26, A=27, etc. The same stratagem is used to create positive differences, such as SARI, 19-1=18 followed by 27-18=9.
"Letter Sums and Differences,"
Word Ways: Vol. 38
, Article 19.
Available at: http://digitalcommons.butler.edu/wordways/vol38/iss4/19