The Vietnamese alphabet is the modern writing system for the Vietnamese language. It uses the Latin script based on Romance languages, in particular, the Portuguese alphabet, with some digraphs and the addition of nine accent marks or diacritics – four of them to create sounds and the other five to indicate tone. These many diacritics, often two on the same vowel, make written Vietnamese recognizable among localized variants of Latin alphabets.
There are 29 “letters” in the Vietnamese alphabet. There are six tones, which are marked in the IPA as suprasegmentals following the phonemic value.
Vietnamese uses 22 letters of the basic Latin alphabet plus 6 additional “letters” where 4 letters are with diacritics: Ă/ă, Â/â, Ê/ê, Ô/ô, Ơ/ơ, Ư/ư, and the letter Đ/đ except for F/f, J/j, W/w, and Z/z. The aforementioned 4 letters are only used to write loanwords, languages of other ethnic groups in the country based on Vietnamese phonetics to differentiate the meanings or even Vietnamese dialects.
The alphabet is largely derived from the Portuguese, although the usage of gh and gi was borrowed from Italian (compare ghetto, Giuseppe) and that for c/k/qu from Greek and Latin (compare canis, kinesis, quō vādis), mirroring the English usage of these letters (compare cat, kite, queen).
The correspondence between the orthography and pronunciation in Vietnamese is somewhat complicated. In some cases, the same letter may represent several different sounds, and different letters may represent the same sound. This is partly due to the fact that the orthography was designed centuries ago and the spoken language has changed.
As mentioned above, Vietnamese is a tonal language, i.e., the meaning of each word depends on the “tone” (basically a specific tone and glottalization pattern) in which it is pronounced. There are six distinct tones in the standard northern dialect. They are called ngang (no tone), sắc (upward/rising tone), huyền (downward/lowering tone), hỏi (lower-rising tone), ngã (lower-rising broken tone), nạng (lowest/abrupt tone).
In the south, there is a merging of the hỏi and ngã tones, in effect leaving five basic tones. The first one (“level tone”) is not marked and the other five are indicated by diacritics applied to the vowel part of the syllable. The tone names are chosen such that the name of each tone is spoken in the tone it identifies.