The development of the original diphthongs in Hittite has seemed to be a peculiarly difficult problem; but evidence has gradually accumulated until something like a system has emerged. We may now say that all the original short diphthongs appear in Hittite as monophthongs, whereas the original long diphthongs retain their diphthongal character. It is not always possible to distinguish with certainty between the several diphthongs; but it will promote clearness if I treat them separately, as far as possible.