Detailed:
University was originally Article 42 of the Book of Rites, Xiao Dai. In the Song Dynasty, Cheng Hao and Cheng Yi brothers took them out of the Book of Rites and compiled chapters and sentences. The Doctrine of the Mean is also an article in The Book of Rites of Little Dai, written by Zi Si, a descendant of Confucius. Zhu named The Great Learning, The Doctrine of the Mean, The Analects of Confucius and Mencius as four books. Since then, Daxue and The Doctrine of the Mean have become Confucian classics.
The Book of Rites:
Eighty-five pieces of Selected Works of Great Virtue were called "Dai Dai Li Ji", but in the later circulation process, only thirty-nine pieces were left in the Tang Dynasty. Dai Shengxuan's forty-nine articles are the Book of Rites of Little Dai that we see today. These two books have their own emphases and choices, and each has its own characteristics. At the end of the Eastern Han Dynasty, Zheng Xuan, a famous scholar, made an incisive comment on The Book of Rites of Little Dai. Later, this book became popular, and gradually became a classic from the works explaining scriptures. The Tang Dynasty was listed as one of the "Nine Classics" and the Song Dynasty was included in the "Thirteen Classics", which is a must-read book for scholars.