29. Two syllables of the same word, not separated by a consonant (especially if the first ends in a short open vowel), are often united into one. The general rules observed in contraction are:
a. Two like vowels unite into their long, εε becoming ει and οο becoming ου (that is, the simple digraphs; cp. § 27).
b. A short vowel disappears in a following diphthong that begins with the same vowel or with the corresponding long.
c. An ο-sound (ο, ω, ου, οι) prevails over an a- or e-sound (α, ᾱ, ε, η, ει), οε and εο becoming ου.
d. Of a- and e-sounds the one that precedes prevails.
e. An open vowel and a close vowel make a diphthong.
b. If the accent was on the first of these, the contract syllable has the circumflex; if on the second, the acute.