Like many Indic scripts, Thai assigns an inherent vowel to each consonant. The pronunciation of the inherent vowel is determined by its position in the word. In an open syllable, the inherent vowel is [a], but in a closed syllable (where the vowel sound is followed by another consonant) it is [o]. The inherent vowel can be modified by means of dependent vowel signs. Long vowels are indicated by a vowel sign above, below or alongside the consonant. Short vowels are represented either by adding a final "short" marker to the long vowel sign, or with their own unique symbol.

Vowels are always written dependently; there is an orthographic rule in Thai that all syllables must begin with a consonant letter. However, spoken Thai does allow syllables to start with a vowel. To allow representation of this, a zero consonant, transcribed in the IPA as a glottal stop, is written as the first letter in a (phonetically) vowel-initial syllable.