In current practice, at least four writing systems are used to write Hmong. The principal and most widely used Hmong orthography is the Romanized Popular Alphabet (RPA), which was devised in the 1950s by a group of missionaries. The RPA exclusively employs the symbols of the Roman alphabet and makes use of a small inventory of notational innovations. The consonant characters employed are largely influenced by the Vietnamese writing system. In addition, vowels followed by the [ng] sound appear doubled (e.g. aang). With the exception of the Mid tone, which is unmarked in the orthography, the six remaining surface tones in Hmong are represented by means of distinct letters written at the end of each syllable. For example, High tones are indicated by placing the letter b at the end of the syllable (e.g. [í] is written ib); Low tones are represented by placing the letter s in a syllable-final position (e.g [ì] is written is); High-Falling tones are marked by means of the letter j (e.g. [í`] is written ij), etc.
The remaining three Hmong orthographies are Phahawh, the Pollard Script, and Chinese Romanized Hmong. Phahawh was developed by a non-literate peasant and is completely unrelated to the other writing systems. The Pollard Script was devised at the turn of the twentieth century by Samuel Pollard to write A-Hmao, a Western Hmong dialect closely related to White/Green Hmong. Although the script is widely known, it is mainly used by Christian-speaking Hmong populations who speak A-Hmao. The Chinese Romanized alphabet was devised following the Communist revolution. It is based on the Pinyin system used to write Mandarin Chinese. However, unlike Pinyin, tones are represented by means of syllable-final letters as in the RPA.