The Tangyun 唐韻 "Rhyme [dictionary] from the Tang dynasty 唐 (618-907)" is a revision and extension of the Sui period 隋 (581-618) dictionary Qieyun 切韻. The characters are arranged according to pronunciation, in rhyme groups. It was compiled by the Tang period scholar Sun Mian 孫愐 during the 730s. It was submitted to the court and adopted as official dictionary. The book is lost but it is known that it was 5 juan "scrolls" long and the characters were arranged according to 195 rhyme groups.
Compared to the Qieyun the Tangyun was a lot more concrete in the field of explaining the characters' meaning and shape, and had thus a better quality as a real "dictionary" in the modern sense. In 1908 a Tang period manuscript fragment of the Tangyun was discovered in Beijing by Jiang Fu 蔣斧. Scholars do not have a common sense about this fragment, some say it is a part of a preface of the Guangyun 廣韻, quoting from the Tangyun, other interpret it as an enlarged version of the Tangyun because it contains more rhyme groups than the original, perhaps as many as 204, which is the number of rhyme groups adopted as standard after the compilation of the Guangyun.
Source: Cao Shujing 曹述敬 (1988). "Yunshu 韻書", in: Zhongguo da baike quanshu 中國大百科全書, Yuyan wenzi 語言•文字, pp. 505-508. Beijing/Shanghai: Zhongguo da baike quanshu chubanshe.