Quoc-ngu, writing system used for the Vietnamese language. Quoc-ngu was devised in the mid 17th century by Portuguese missionaries who modified the Roman alphabet with accents and signs to suit the particular consonants, vowels, and tones of Vietnamese. It was further modified by a French missionary, Alexandre de Rhodes. At first used only in Vietnamese Christian communities, it was made compulsory by the French administration in 1910. Quoc-ngu is now universally used in Vietnam and is the official writing system.