How did writing systems for Hmong-Mien languages develop in Hunan?

Historically, Hmong-Mien languages in Hunan and other parts of China were unwritten. The development of writing systems for these languages primarily occurred during the 20th century through two main approaches:

• Missionary Scripts: Early efforts included the invention of the Pollard script by missionary Samuel Pollard, which used primary symbols for consonants and secondary symbols for vowels, with tone indicated by the placement of vowel symbols relative to consonants.12
• Phonetic Romanization: Since 1949, these earlier scripts have been largely revised, extended, or replaced by phonetic systems based on the Latin alphabet. These systems are akin to the Pinyin romanization used for Mandarin Chinese, with letter-to-sound relationships following Pinyin conventions.21

These romanized systems are designed to account for the tonal nature of the languages by using final consonant symbols to represent tone rather than actual consonant sounds, as final consonants are rare in these languages.1