hannah+egg

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This new type of verse forms was lü shih, or regulation poetry.
Regulation poetry was built upon two rhetorical devices: (1) parallel structure facilitated by the monosyllabic pronunciation of each graph, ensuring easy recognition of the parallelism when the passage is read aloud, and enhancing the appeal to the eye when the passage is recorded in characters, each occupying exactly the same amount of space; and (2) intonation which assigns a specific tone or pitch to each word, and which, therefore, may increase the euphony of the sentence or line of poetry by a tested arrangement of words differently intonated in proper sequence.

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Originally resorted to mainly for the purpose of remedying the paucity of vocables in the Chinese language and of avoiding confusion between homonyms, intonation had grown in complexity before T’ang times also as a result of the introduction of Buddhist incantations and Indic phonology. By the seventh century, there were already in the language spoken in metropolitan Ch’ang-an, the T’ang imperial capital, eight different tones. These were, arranged in four pairs, as follows:

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Without going into the complexities and niceties of T’ang and post-T’ang prosody, all that we need to bear in mind is that they were grouped by T’ang poets into only two categories, namely, the even tones (1,2) and the deflected tones.