The numerical positions of all the vowels in the alphabet are odd numbers. Designating their placement by number, we get A=1, E=5, I=9, O=15, U=21 and Y=25. Thus, only words that have no vowels, like NTH (14-20-8) can be written with letters whose numeric alphabetic position is strictly even.
"Mississauga, Canada's Oddest City,"
Word Ways: Vol. 25
, Article 15.
Available at: http://digitalcommons.butler.edu/wordways/vol25/iss1/15