A diphthong (δίφθογγος having two sounds) combines two vowels in one syllable. The second vowel is ι or υ. The diphthongs are: αι, ει, οι, ᾱͅ, ῃ, ῳ; αυ, ευ, ου, ηυ, and υι. The ι of the so-called improper diphthongs, ᾱͅ, ῃ, ῳ, is written below the line and is called iota subscript. But with capital letters, ι is written on the line (adscript), as ΤΗΙ ΩΙΔΗΙ = τῇ ᾠδῇ or Ὠιδῇ to the song. All diphthongs are long.
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.