In the study of Heian-era hentai-kambun (a style based mainly upon Japanized Chinese), one possible approach is to follow historical changes in expressive forms of opening and final parts of the sentences.
At Buddhistic congregation the script called Ganmon on which the president praied for the repose of a dead person's soul was announced, and the main body of ganmon consisted of couplets. Most of ganmon were written by the then stylists.
Nearly 200 ganmon samples were examined for this analysis.
I reach the conclusion that the samples show a tendency toward fixed forms of opening and final parts in the middle part of the Heian Era.