What is the article called "Double Wall" in the history of ancient poetry in China?

"Yuefu Shuangbi" is a metaphor for Mulan's poems and peacocks flying southeast. Mulan Poetry, also known as Mulan Ci, is a folk song of the Northern Dynasties. Peacock Flying Southeast, also known as Ancient Poems for Jiao Zhongqing's Wife, is one of the representative works of ancient Yuefu folk songs and the earliest existing long narrative poem.