U+1ED1 Latin Small Letter O with Circumflex and Acute

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The Vietnamese alphabet (Vietnamese: chữ Quốc ngữ, lit. 'Characters/script of the national language', IPA: [t͡ɕɨ˦ˀ˥ kuək̚˧˦ ŋɨ˦ˀ˥]) is the modern writing script for Vietnamese. It uses the Latin script based on Romance languages originally developed by Francisco de Pina (1585–1625), a missionary from Portugal.

The Vietnamese alphabet contains 29 letters, including seven letters using four diacritics: ⟨ă⟩, ⟨â⟩, ⟨ê⟩, ⟨ô⟩, ⟨ơ⟩, ⟨ư⟩, and ⟨đ⟩. There are an additional five diacritics used to designate tone (as in ⟨à⟩, ⟨á⟩, ⟨ả⟩, ⟨ã⟩, and ⟨ạ⟩). The complex vowel system and the large number of letters with diacritics, which can stack twice on the same letter (e.g. nhất meaning 'first'), makes it easy to distinguish the Vietnamese orthography from other writing systems that use the Latin script.

The Vietnamese system's use of diacritics produces an accurate transcription for tones despite the limitations of the Roman alphabet. On the other hand, sound changes in the spoken language have led to different letters, digraphs and trigraphs now representing the same sounds.