A vowel, by definition, is a sound produced by the vibrations of the vocal cords without the friction or stoppage which characterizes consonants. The alphabetical vowels are A, E, I, O, and U, comprising about 40 percent of the letters in all written words. Actually, A and I are dipthongs; one vowel sound immediately followed by another I is pronounced ah-ee.
Word Ways: Vol. 18
, Article 15.
Available at: http://digitalcommons.butler.edu/wordways/vol18/iss2/15