TRIPHTHONGS BEGINNING WITH I.
The triphthongs ieu and iew both sound like open or long u; as in lieu, adieu, view.
The three vowels iou, in the termination ious, often fall into one syllable, and form a triphthong. There are two hundred and forty-five words of this ending; and more than two hundred deriva- tives from them. Walker has several puzzling inconsistencies in their pronunciation; such as fas-tid-i-ous and per-fid-ious, con-ta-gi-ous and sac-ri-le-gious. After c, g, t, or x, these vowels should coalesce: as in gra-cious, re-li-gious, vex-a-tious, ob-nox-ious, and about two hundred other words. After the other consonants, let them form two syllables; (except when there is a syn- seresis in poetry;) as in dw-bi-ou-s, o-di-ous, va-ri-ous, en-vi-ous.