本文根據詩經和離騷韻律結構的差異提出兩者分屬兩種不同的韻律體系:詩經以雙音節音步建立的二步律(dipodic prosody)為基礎，而離騷則屬靈活多變的頓歎律(caesura prosody)。文章認為，西漢以降，隨情而異的頓歎律逐漸取代了音節固定的二步律，於是先三音節、後五七音節的詩行相繼而見。然而，為什麼只有奇數音節的詩行得到發展的原因及其如何發展的機制，迄今沒有很好的解釋。本文即在嘗試回答這些問題的基礎之上，提出古代詩體的形成與演變，是由當時語言的韻律語法所決定的觀點與分析。

This paper proposes an independent system of prosodic application used in Lisao 離騷(attr. Qu Yuan, c.340-278 B.C.), different from the one in Shijing 詩經 (Book of Odes). It is shown that while a fixed dipodic prosody (二步律) was used in Shijing, a caesura-based prosody(頓歎律)was developed in Lisao during and after the Warring States period (475-221 B.C.). By the Han dynasty (206 B.C.-220 A.D.) however, flexible caesura prosody was gradually replaced by syllabically-fixed patterns: first trisyllabic, then pentasyllabic and finally heptasyllabic. To date, there has been no reasonable explanation proposed for why and how only odd numbers of syllables in lines developed. This paper deals with this question from a prosodic point of view. It is argued here that poetic evolutions in classical Chinese are determined by the grammar of poetic prosody.