Term jat developed from preindoeuropean monophthongal long e and monophthongization preslavonic diphtong /ai/ and /oi/ in some positions. In preslavonic texts at the beginning of the word and behind the vocal pronunciation is reconstructed /ja/, a in positions behind the consonants of national philology they act differently. Its croatian tradition that the jat accomplishes itself as in contemporary crotian standard language like /ije/je/. Jat has experienced the most complex development in speeches of štokavica ijekavica, which are the foundation of the contemporary croatian literary language. In those speeches during the middle croatian period from closed /e/ diphtong /ie/ evolves into long and short syllables.Pronunciation of the long jat in speeches of new štokavica can be monosyllabic (dvoglasnički ili troglasnički) and disyllabic (trofonemski). Speakers of the croatian literary language had never completely adopted the disyllabic pronunciation of long jat and so the pronunciation is mostly monosyllabic as in western speeches of štokavica and ijekavica that were the foundation of the type of ijekavica of the croatian language standard somewhere up to the middle of the 18th century and to the Ilyrian period. So did the croatian expressional, usable and standard norm differed for a long time from the one grammatically codified. Today is grammatically codified, but the diphtong ie in the writing is still noted as ije. Nevertheless that the contemporary croatian standard norm is mostly codified, the problems and polemisations around jat, its writing and pronunciation still spring (arise) today. In the teaching of croatian language, primarily the reflection of jat bestowes the biggest problems and hesitations besides writing of č and ć and đ and dž.