In letter-shift word pairs, the letters in one word are an equal number of alphabetic steps away from the corresponding letters in the other word. CHEER to JOLLY is perhaps the most well-known example, with a letter-shift value of seven (C to J, H to O, etc.). FAT to NIB with a letter-shift value of eight works the same way, except the shift from T to B involves counting letters on a circular alphabet where Z is followed by A. Because the letters are equidistant, letter-shift words can be considered parallel to each other.
"The Word Calculator,"
Word Ways: Vol. 21
, Article 17.
Available at: http://digitalcommons.butler.edu/wordways/vol21/iss2/17