Māori has many doublets like /raŋo/ = /ŋaro/ (from Proto-Polynesian *laŋo) and /pouaru/ (North Island) = /poueru/ (South Island). Many of them occur due to metathesis, or the rearranging of sounds. In Māori's case metathesis switches adjacent vowels, consonants or syllables; in addition to that there exists a rare type of metathesis that involves sound features instead of segments: in tenga ~ kenakena ('Adam's apple') the consonants' place of articulation changes while retaining nasality; in inohi ~ unahi ('scales') the subject of metathesis is the vowel labialisation, but not the vowel height. Some morphemes have allomorphs: for example, the prefix /ŋaːti/ changes to /ŋaːi/ if it is preceding a word that starts with /t/: /ŋaːti porou/, but /ŋaːi tahu/; the same can be observed for /motu/ ('island'): /moutohoraː/, Moutohora Island.