In the pre-Qin period, the word "Xi" was only used in poetry and a few rhyming styles, but not in prose.
As we all know, general auxiliary words or modal particles can be used in prose and verse, such as Hu, Ye, Yan and Zai, and their usage basically reflects the reality of daily life language.
The word "Xi" is dedicated to poetry, which shows that its function is closely related to the characteristics of poetic language.
The word "Xi" appeared in a large number of pre-Qin poems and fu, and its pronunciation was similar to "ha" and "ah", which was still widely used in Han Dynasty. Chu Ci was popular in the early Han Dynasty, copied by many scholars and widely used in Han Fu. After the Han dynasty, with the continuous evolution of poetic style, the word "Xi" is used less and less in poetry.