Tamil phonology is characterised by the presence of "true-subapical" retroflex consonants and multiple rhotic consonants. Its script does not distinguish between voiced and unvoiced consonants; phonetically, voice is assigned depending on a consonant's position in a word, voiced intervocalically and after nasals except when geminated. Tamil phonology permits few consonant clusters, which can never be word initial.
Vowels
The vowels are called ('life letter'). The vowels are classified into short and long (five of each type) and two diphthongs.
The long (netil) vowels are about twice as long as the short (kuṟil) vowels. The diphthongs are usually pronounced about 1.5 times as long as the short vowels, though most grammatical texts place them with the long vowels.
Tamil has two diphthongs: and , the latter of which is restricted to a few lexical items. Some like Krishnamurti consider the diphthongs as clusters of /a/ + /j, ʋ/ as they pattern with other VC. The way some words are written also varies e.g. avvai as (avvai), (auvai) or (avvay) (first one most common). Word final is pronounced as , it is called a (kuṟṟiyalukaram) "short u" (as it has only half a sound unit, compared to 1, 1.5 or 2 of other vowels) in tolkāppiyam and it is unrounded even in literary Tamil; in spoken Tamil it can occur medially as well in some words after the first syllable. Word final [u] occurs in some names, chiefly male nicknames like rājēndraṉ as rāju.
Almost all forms of colloquial Tamil add different vowel sounds for many of the kuṟil vowels. For example, , instead of making an sounds, often produces a sound instead, except at the end of words, where it always produces an sound. Similarly, the letter often produces an /ɛ/ sound (except at the end of the word). At the end of words, the letter உ often produces an /ɯ/ sound, except for names and nicknames. The letter அ almost always produces a /ə/ sound, or a sound similar.
Colloquially, an initial or may have a onglide; likewise, an initial or may have a onglide, e.g. and . This does not occur in Sri Lankan dialects.
Indian Colloquial Tamil also has nasalized vowels formed from word final vowel + nasal cluster (except for /Vɳ/ where an epenthetic u is added after it). Long vowel + nasal just nasalizes the vowel, short vowel + nasal may also change the quality, for example, /an/ gets fronted to [ɛ̃] becomes [aʋɛ̃] ([aʋæ̃] for some speakers), /am/ gets rounded to [õ] becomes , becomes , becomes , the remaining vowels only get nasalized.
In spoken Tamil sometimes an epenthetic vowel u is added to words ending in consonants, e.g. nil > nillu, āḷ > āḷu, nāḷ > nāḷu (nā in some dialects), vayal > vayalu etc. If another word is joined at the end, it is deleted.
Colloquially, the high short vowels , are lowered to and when next to a short consonant and . For example, becomes ; and becomes . It doesn't happen in pronouns and some other words e.g. ivaṉ and evaṉ are different words. also monophthongises to an but it causes the lowering of before it, e.g. ilai > ele. Additionally, the front long vowels and are subject to retraction when present in the first syllable of a bisyllabic word and followed by a retroflex consonant. As such, "house" becomes , but its inflected form remains . Likewise, "search!" becomes , but "(he) searched" remains . The presence and degree of retraction for each vowel may be different; it varies between dialects and even individual speakers. Almost all words end with vowels in spoken Tamil.
For some speakers in spoken Tamil the front vowels /i(:), e(:)/ get rounded to their corresponding rounded back vowels when they are after a labial consonant /m, p, ʋ/ and before a retroflex consonant, some words with it are quite acceptable like பெண் /peɳ/ > பொண்/பொண்ணு [poɳ~poɳ:ɯ] but others like வீடு /ʋi:ʈu/ > வூடு [ʋu:ɖɯ] are less accepted and may even be considered vulgar.
Another change in spoken Tamil is vowel harmony, where vowels change their height to be more similar to nearby vowels: e.g. literary Tamil /koʈu/ > spoken Tamil [kuɖɯ].
Consonants
The consonants are known as ('body letters'). The consonants are classified into three categories with six in each category: valliṉam ('hard'), melliṉam ('soft' or nasal), and iṭayiṉam ('medium'). Tamil has very restricted consonant clusters (for example, there are no word-initial clusters). There are well defined rules for voicing stops in the written form of Tamil, Centamiḻ (the period of Tamil history before Sanskrit words were borrowed). Stops are voiceless when at the start of a word, in a consonant cluster with another stop and when geminated. They are voiced otherwise.