The vowels are generally separated into two categories&colon; monophthongs and diphthongs. Vowels occur as simple peaks or as peak nuclei in diphthongs. The Polish vowel system consists of six oral monophthongs and two nasal diphthongs. In Belgian Dutch, there are also fewer vowels pronounced as diphthongs. Kazakh has a system of 12 phonemic vowels, 3 of which are diphthongs. There are special vowel markings for certain diphthongs. Certain sound changes relate to diphthongs and monophthongs. Brahui vowels show a partial length distinction between long and diphthongs and short . This includes the so-called "mixed diphthongs", composed of a short vowel followed by a sonorant. For more detailed explanations of Gaelic diphthongs see Scottish Gaelic orthography. Phonetically, Spanish has seven falling diphthongs and eight rising diphthongs. They tend to be higher than the ending points of the phonemic diphthongs . The diphthongs of some German dialects differ from standard German diphthongs. A full list of sounds, diphthongs, and their main spellings is given at Portuguese phonology. The amount of diphthongs varies significantly between dialects.