and are spelled iê, ươ, uô, respectively, when they are followed by a consonant. In addition to single vowels (or monophthongs) and centering diphthongs, Vietnamese has closing diphthongs and triphthongs. The closing diphthongs and triphthongs consist of a main vowel component followed by a shorter semivowel offglide or . There are restrictions on the high offglides: cannot occur after a front vowel (i, ê, e) nucleus and cannot occur after a back vowel (u, ô, o) nucleus. {| class="wikitable" style="text-align: center;" |- ! rowspan="2"| ! colspan="2"| offglide ! colspan="2"| offglide |- ! Front ! colspan="2"| Central ! Back |- ! Centering | iêu | ươu | ươi | uôi |- ! Close | iu | ưu | ưi | ui |- ! Close-mid/Mid | êu | –âu | ơi ây | ôi |- ! Open-mid/Open | eo | ao au | ai ay | oi |} The correspondence between the orthography and pronunciation is complicated. For example, the offglide is usually written as i; however, it may also be represented with y. In addition, in the diphthongs and the letters y and i also indicate the pronunciation of the main vowel: ay = ă + , ai = a + . Thus, tay "hand" is while tai "ear" is . Similarly, u and o indicate different pronunciations of the main vowel: au = ă + , ao = a + . Thus, thau "brass" is while thao "raw silk" is . Consonants The consonants that occur in Vietnamese are listed below in the Vietnamese orthography with the phonetic pronunciation to the right. {| class=wikitable style=text-align:center |- ! colspan=2| ! Labial ! Dental/Alveolar ! Retroflex ! Palatal ! Velar ! Glottal |- ! colspan=2| Nasal | m | n | | nh | ng/ngh | |- ! rowspan=3| Stop ! tenuis | p | t | tr | ch | c/k/q | |- ! aspirated | | th | | | | |- ! implosive | b | đ | | | | |- ! rowspan=2| Fricative ! voiceless | ph | x | s | | kh | h |- ! voiced | v | d/gi | | | g/gh | |- ! colspan=2| Approximant | | l | | y/i | u/o | |- ! colspan=2| Rhotic | | colspan=2| r | | | |} Some consonant sounds are written with only one letter (like "p"), other consonant sounds