Characters

Vowels: உயிர் எழுத்து (uyir eḻuttu, "soul letters")

Twelve vowels, split the way Sanskrit's are: five short and long pairs, plus two diphthongs.

Short (குறில், kuṟil).

Long (நெடில், nedil): about twice the length of the short vowel, like Sanskrit's macron pairs.

Diphthongs: about one and a half times the length of a short vowel.

Consonants: மெய் எழுத்து (mey eḻuttu, "body letters")

Twelve of the eighteen line up in the same place-of-articulation grid Sanskrit uses: one row of stops (வல்லினம், vallinam, "hard") and one row of matching nasals (மெல்லினம், mellinam, "soft"). There are only two rows, not five, because Tamil's stops have no separate voiced or aspirated letters. க், for instance, is just /k/. The same letter is pronounced [g] or [x] depending on where it falls in a word, where Sanskrit would write a different letter (க/ग/ख/घ).

Letters for Sanskrit & other loanwords: கிரந்த எழுத்து (grantha)

Six more letters, borrowed from the Grantha script that was once used to write Sanskrit across the Tamil country, cover sounds that Tamil's native eighteen don't distinguish. The clearest case is the difference between Tamil's single plain ஸ/ச-type sound and Sanskrit's separate ś, ṣ, s and h. Children learn these letters from elementary school as part of the everyday alphabet. They are not treated as foreign.

Tamil almost never fuses two consonants into a single conjunct glyph the way Devanagari does. The standard exceptions are க்ஷ (kṣa) and the ligature ஶ்ரீ (śrī). Vel Maaral's colophon spells the same word ஸ்ரீ instead, with ஸ் (plain s) in place of ஶ் (ś). Both spellings are in ordinary use, since spoken Tamil doesn't distinguish the two sounds either way.

Building a syllable

Every consonant is written with its vowel suppressed. A small mark called புள்ளி (puḷḷi) does this: the dot on க் above. Combine the consonant with one of the twelve vowels and the puḷḷi is replaced by that vowel's own attached form, giving a new glyph for the syllable. This is the same "consonant + vowel = one shape" idea as Sanskrit's mātrā signs, but carried through the full vowel set instead of a subset.

க் (k) run through all twelve vowels. Each of Tamil's eighteen consonants makes the same twelve forms, giving 18 × 12 = 216 combination letters. Add the 12 vowels, the 18 bare consonants and ஃ, and you get the 247 characters traditionally counted as "the Tamil alphabet". Below is that full set, laid out consonant by consonant so you can check க's pattern against the rest.

Vallinam & Mellinam: வல்லினம் & மெல்லினம்

Idaiyinam: இடையினம்