Yueshu yaolu 樂書要錄 "Important records about writings on music" is a compendium on music compiled on imperial order of Empress Wu Zetian 武則天 (624-705) and under the supervision of Yuan Wanqing 元萬頃 (c. 640-690). He also compiled the books Lienüzhuan 列女傳 (a new edition), Chengui 臣軌 and Bailiao xinjie 百寮新戒.
The book of 10 juan was finished in 700 CE, but was lost soon. Yet a copy had been taken to Japan by the scholar Kibi no Makibi 吉備真備 (695-775) in 735. Of this specimen, only juan 5 to 7 have survived. These fragments returned to China in the late 19th century. The surviving part of the Yueshu yaolu gives an impression of the high standard of music during the Tang period 唐 (618-907). It differed from that of earlier ages, but also from the practice of music during the Song period 宋 (960-1279).
The fifth fascicle consists of 10 chapters of a theoretical nature. The first chapter (Bian yinsheng shensheng yuan 辨音聲審聲源) includes a quotation from Cai Yong's 蔡邕 (132-192) text Yueling zhangju 月令章句 "Paragraph-and-sentence commentary to the Yueling", where it is said that in ancient times people used to tune the pitchpipes by ear (yi er qi qi sheng 以耳齊其聲), in addition to mathematical calculations concerning the length and volume of the pipes. In the chapter Lun er bian yi 論二變義 the alteration of the two tones gong 宮 and zhi 徵 are explained.
Other chapters of juan 5 are also dedicated to this theme, like Qi sheng xiang sheng fa 七聲相生法 (describing the relation of each tone to the preceding one, in a system similar to the phenomena of the calendar, li ba xiangsheng yi 曆八相生義), Lun xiang sheng lei lie 論相生類例 (discussing the position of the keynote tone of a scale) or Qi sheng ci di yi 七聲次第義. The chapter Lun sanfen sunyi tong zhu xian guan 論三分損益通諸弦管 discusses the method of producing a new tone in the scale by adding or deducting a third to or from the length of the pipe or string. In the final chapter, Yuepu 樂譜, the use of pentatonic music in daily life is dealt with.
The sixth fascicle describes the system of the musical modes and pitchpipes and quotes extensively from older writings, like the treatise on astronomy and music Lüli zhi 律曆志 in the official dynastic history Hanshu 漢書, the Yueling zhangju or Sanli yizong 三禮義宗. It includes four chapters describing the dimensions of the pitchpipes and the lengths of strings and their relations to the universe (Ji lülü 記律呂), "harmonious chant" (Qiankun changhe yi 乾坤唱和義), observing the "right weight" (Jin quanliang 謹權量, where the dimensions of the pitchpipes are related to other measures) and the correlation of the pipes with the seasons (Shen feihou 審飛候).
The seventh fascicle includes three chapters on modulation (Lülü xuangong fa 律呂旋宮法), harmony (Shisheng lüfa 識聲律法) and the seven musical keys (Lun yi lü you qi sheng yi 論一律有七聲義). The sequence of modulating the keys is described, in clockwise (shunxuan 順旋) and counter-clockwise (nixuan 逆旋) direction. This is the first instance in Chinese literature on music where modulation is concretely described.
The Yueshu yaolu included five charts illustrating the harmonious "birth" of the next tone in a scale (Qiankun changhe tu 乾坤唱和圖), the method of enclosing the circle of the twelve pipes built up in the central one of three sealed rooms to analyze their relation to the months (Han lüshi tu shi'er lü xiang sheng tu 漢律室圖十二律相生圖), and illustrations to the seven different tonal positions of an individual pipe (Yi lü you qi sheng tu 一律有七聲圖).
Quotations from the other parts of the Yueshu yaolu are to be found in Japanese texts on music, but until now no systematic collection of these fragments has been made. The book Sango yōroku 三五要錄, for instance, quotes from the modulation of the pipa lute (pipa xuangong fa 琵琶旋宮法), and the book Agetsu mondō 阿月問答 from juan 10. The text Onritsu tsūchi shō 音律通致章 gives an overview of the central chapters of the Yueshu yaolu. Fragments can also be found in the books Shōmei yōshin shū 聲明用心集, Ruihitsu jiyō 類筆治要 and Taigenshō 體源抄.
Two important Japanese studies on the Yueshu yaolu were written by Hazuka Keimei 羽塚啓明 (Gakusho yōroku kaisetsu 楽書要録解説 and Kōi Gakusho yōroku 校異楽書要録).