Two Morae, and Why the Inscriptions Keep Them Apart

Vowel length in these languages is quantity, measured in morae, and the count decides the shape of a line of verse. A syllable is heavy if its vowel is long, or if a cluster of consonants follows a short one, and the classical metres are patterns of heavy and light syllables rather than of stress. A scribe who lost the distinction between the two a vowels would make the verse unreadable as verse, and that is one reason the mark for the long vowel is kept distinct even in rapidly cut inscriptions where other letters are formed loosely.

The distinction survives the whole descent of the script. Every alphabet that comes out of Brahmi keeps a separate sign for the long vowel, in both its independent and its attached form, and keeps it in second place in the recited order. It survives even in the Prakrit of the Ashokan edicts, where a good deal else of the older sound system had already fallen together, because length remained meaningful in Middle Indo-Aryan. Where the descendants differ from one another is in the shape of the mark rather than in whether they have one. Comparing that one mark across the regional hands of the centuries after the Mauryan period is a routine method of Indian palaeography, since a sign this common gives a palaeographer plenty of examples to date an undated inscription against.