Changes in the script were made by French scholars and administrators, and by conferences held after independence during 1954-1974. The script now reflects a so-called Middle Vietnamese dialect which has vowels and final consonants most similar to northern dialects and initial consonants most similar to southern dialects. (Nguyễn 1996). This Middle Vietnamese is presumably close to the Hanoi variety as spoken sometime after 1600 but before the present.
Prior to French rule, the first two Vietnamese writing systems were based on Chinese script:
~ The standard ideographic Chinese character set called chữ nho
(scholar's characters, 字儒): used to write Literary Chinese
~ A complicated variant form known as chữ nôm (southern/vernacular characters, 字喃) with characters not found in the Chinese character set; this system was better adapted to the unique phonetic aspects of Vietnamese which differed from Chinese
The authentic Chinese writing, chữ nho, was in more common usage, whereas chữ nôm was used by members of the educated elite (one needs to be able to read chữ nho in order to read chữ nôm). Both scripts have fallen out of common usage in modern Vietnam, and chữ nôm is near-extinct.