The form shown here is based on examples from the 7th century AD. Letters labeled * have uncertain sound value, as they have little occurrence in Southeast Asia. (Chart of images based on Pallava glyphs presented at SkyKnowledge.com.)
Each consonant has an inherent /a/, which will be sounded if no vowel sign is attached. If two consonants follow one another without intervening vowel, the second consonant is made into a subscript form, and attached below the first.
Sivaramamurti, C, Indian Epigraphy and South Indian Scripts. Bulletin of the Madras Government Museum. Chennai 1999