Pedanticus wrote:... I was particularly puzzled by the suggestion that a grave accent was used to mark a syllable that was previously (but no longer) acute, and so should be pronounced at base level. ...
The accent marking system is sometimes said to have a simpler precursor: each unaccented syllable after the stress was marked by grave accent mark:
The acute, the only mark then was ommited in such case:
This use is said to be retained in disyllabic prepositions like [face=SPIonic]peri\[/face]. The preposition do not come with acute mark in the text anyway.
Hence the hypothesis could be emendated. The syllable before the grave gains an acute, unless it is marked with other accent mark. The grave marked syllable loses its accent.