A basic shift sees the letters of a word shifted along the alphabet n steps, looping from Z to A, to make another word. Today, however, the alphabet, as we recognise it, no longer exists. The 5 major vowels have opted out of that all-embracing 26-letter series to set up an alphabet of their own, predictably christened the 'vowel alphabet', AEIOU.
"Shifting Vowels & Consonants Independently,"
Word Ways: Vol. 40
, Article 16.
Available at: http://digitalcommons.butler.edu/wordways/vol40/iss2/16