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.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].