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.[7]
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.
^ abJacques, Roland (2002). Portuguese Pioneers of Vietnamese Linguistics Prior to 1650 – Pionniers Portugais de la Linguistique Vietnamienne Jusqu'en 1650 (in English and French). Bangkok, Thailand: Orchid Press. ISBN974-8304-77-9.
^Jacques, Roland (2004). "Bồ Đào Nha và công trình sáng chế chữ quốc ngữ: Phải chăng cần viết lại lịch sử?" Translated by Nguyễn Đăng Trúc. In Các nhà truyền giáo Bồ Đào Nha và thời kỳ đầu của Giáo hội Công giáo Việt Nam (Quyển 1) – Les missionnaires portugais et les débuts de l'Eglise catholique au Viêt-nam (Tome 1) (in Vietnamese & French). Reichstett, France: Định Hướng Tùng Thư. ISBN2-912554-26-8.
^Haudricourt, André-Georges. 2010. "The Origin of the Peculiarities of the Vietnamese Alphabet." Mon-Khmer Studies 39: 89–104. Translated from: Haudricourt, André-Georges. 1949. "L'origine Des Particularités de L'alphabet Vietnamien." Dân Viêt-Nam 3: 61–68.
^Jakob Rupert Friederichsen Opening Up Knowledge Production Through Participatory Research? Frankfurt 2009 [6.1 History of Science and Research in Vietnam] Page 126 "6.1.2 French colonial science in Vietnam: With the colonial era, deep changes took place in education, communication, and ... French colonizers installed a modern European system of education to replace the literary and Confucianism-based model, they promoted a romanized Vietnamese script (Quốc Ngữ) to replace the Sino-Vietnamese characters (Hán Nôm)"