The late Ming and early Qing drama theorist Li Yu (1611-1680) used this term to suggest that a drama should revolve around main characters and events. He said that when an ancient writer wrote an essay, he would always develop a main theme first. This main theme was what he wanted to write about. A drama, too, should unfold around a key character and a key event, namely, the main theme of the drama, which would spur plot development and interaction between characters. The key event was the one which, of all events in the drama, played a central role in developing the plot and which linked together all plots elements. As an example, Li Yu said the main character in Romance of the Western Chamber was Zhang Junrui, whom the White Horse General rescued and around whom all the other characters and events revolved. Main characters and main events were equally important. The authors of many legendary dramas knew how to create a main character but did not know how to set up a main event. As a result, their works looked like broken strings of pearls or houses without pillars. Establishing a main theme was very important to the structure of a drama.

When an ancient writer wrote an essay, he would always develop a main theme first. Such a theme is nothing other than what he intended to convey through the essay. The same is true for dramatic legends.

(Li Yu: Occasional Notes with Leisure Motions)

When writing an essay, one can sum up its purpose in a single sentence and expand it into thousands of sentences. That is what a "main theme" means – it is roughly a single-sentence summary.

(Liu Xizai: Overview of Literary Theories)