Synizesis may occur between two words when the first ends in a long vowel or diphthong. This is especially the case with δή
a. The term synizesis is often restricted to cases where the first vowel is long. Where the first vowel is short, ε, ι were sounded nearly like y; υ nearly like ω. Cp. cross44 a. The single syllable produced by synizesis is almost always long.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].