There are seven vowels: α, ε, η, ι, ο, υ, ω. Of these ε and ο are always short, and take about half the time to pronounce as η and ω, which are always long; α, ι, υ are short in some syllables, long in others. In this Grammar, when α, ι, υ are not marked as long (α_, ι_, υ_) they are understood to be short. All vowels with the circumflex ( cross149) are long. On length by position, see cross144.
a. Vowels are said to be open or close according as the mouth is more open
or less open in pronouncing them, the tongue and lips assuming different positions in the case of each.5
A diphthong (δίφθογγος
a. In ᾳ, ῃ, ῳ the ι ceased to be written about 100 B.C. The custom of writing ι under the line is as late as about the eleventh century.5D
A diphthong ωυ occurs in New Ionic (ὡυτός
ει, ου are either genuine or spurious (apparent) diphthongs ( cross25). Genuine ει, ου are a combination of ε ι, ο υ, as in λείπω
The figure of a triangle represents the relations of the vowels and spurious diphthongs to one another. [unresolved image link]
From α_ to ι and from α to ου the elevation of the tongue gradually increases. ω, ο, ου, υ are accompanied by rounding of the lips.8
Diaeresis.—A double dot, the mark of diaeresis (διαίρεσις
In poetry and in certain dialects vowels are often written apart which later formed diphthongs: πάις (or πάϊς)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].