Vowel nuclei consist of monophthongs (simple vowels) and three centering diphthongs. Furthermore, Ancient Greek diphthongs became monophthongs. Proto-Slavic had eliminated most diphthongs creating either long monophthongs or nasal vowels. Younger speakers may vary between Standard Japanese diphthongs and dialectal monophthongs. Old Irish had distinctive vowel length in both monophthongs and diphthongs. The Polish vowel system consists of six oral monophthongs and two nasal diphthongs. The vowels are generally separated into two categories&colon; monophthongs and diphthongs. The language has both monophthongs and diphthongs, and consonants can be voiced or unvoiced. Certain sound changes relate to diphthongs and monophthongs. The language has both monophthongs and diphthongs, and many consonants can be voiced or unvoiced. All Japanese vowels are purethat is, there are no diphthongs, only monophthongs. Apart from these monophthongs, there are also three diphthongs . In other dialects, they changed to wide range of monophthongs and diphthongs. Like monophthongs, diphthongs appear to have had short and long versions. The diphthongs became the long monophthongs and before the Classical period.