Vietnamese is a tonal language, i.e. the meaning of each word depends on the "tone" in which it is pronounced. Many other languages also use tones, like Chinese.
There are six distinct tones; the first one ("level tone") is not marked, and the other five are indicated by diacritics applied to the main vowel of the syllable:
|Ngã (Rising glottalized)||Tilde||Ã/ã||Ẵ/ẵ||Ẫ/ẫ||Ẽ/ẽ||Ễ/ễ||Ĩ/ĩ||Õ/õ||Ỗ/ỗ||Ỡ/ỡ||Ũ/ũ||Ữ/ữ||Ỹ/ỹ|
|Nặng (Falling glottalized)||Dot below||Ạ/ạ||Ặ/ặ||Ậ/ậ||Ẹ/ẹ||Ệ/ệ||Ị/ị||Ọ/ọ||Ộ/ộ||Ợ/ợ||Ụ/ụ||Ự/ự||Ỵ/ỵ|
The lowercase letter "i" should retain its dot even when accented. (However, this detail is often lost in computers and on the Internet, due to the obscurity of Vietnamese specialty fonts and limitations of encoding systems.)