My editor informed me that, if I use any diaeresis marks at all, then I must be consistent! I consistently use a diaeresis mark in coördinator and coöperation.
The periodical, Popular Educator (c 1890) had this to say:
“The Diæresis: [#] 77. The diæresis shows that the letter over which it is placed is to be pronounced separately; as, Creätor, zoönomia, aërial … “
I agree with, zo-onomia; and I can understand, Cre-ator, but a-erial?
The Encarta Dictionary (c 2000) shows the pronunciation of (aerial) as áiree əl.
Is it just my accent, or is there a hiatus in aerial?