In the High Tang 盛唐 (roughly from 713 to 755), just as the pentasyllabic-line poetic form lay firmly under the sway of the Song-Qi 宋齊 (420–502) style, Du Fu 杜甫 (712–770) arrived at a solution to the problem of restoring to the mid-to-long wugu 五古 (ancient-style pentasyllabic) poem the old tenor of the Han 漢 (202 BCE–220 CE) and Wei 魏 (220–65) periods. However, Du Fu did not merely imitate his Han-Wei predecessors in terms of form and style. Instead, with his efforts at recovery of the creative principles driving the Han-Wei ancient form, Du Fu succeeded in distilling a novel wugu rhythm from the life and language of his own time. By its exploitation of the potential of the wugu form to narrate by single prose lines, by its enhancement of both the density of narrative sequences and the compactness of the rhythm of lines and passages, and by its exploration of the use of various modes of narration within a poem, Du Fu's work shows a key distinguishing feature we might term shi zhong you wen 詩中有文, or “prose within poem.” At the same time, in his mid-to-long poems, Du Fu was able both to bend the inherent capacity of the wugu for narration to its greatest expressive potential and also, by the use of expressive rhythm as a guiding force, to strictly maintain the fundamental distinction between poetry and prose. In considering why Du Fu's work is considered shishi 詩史 (history in poetry), we must recognize the critical formal expedient provided by this important creative breakthrough in the wugu form.