All Chinese characters are logograms, but several different types can be identified, based on the manner in which they are formed or derived. There are a handful which derive from pictographs () and a number which are ideographic () in origin, including compound ideographs (), but the vast majority originated as phono-semantic compounds (). The other categories in the traditional system of classification are rebus or phonetic loan characters () and "derivative cognates" (). Modern scholars have proposed various revised systems, rejecting some of the traditional categories. In older literature, Chinese characters in general may be referred to as ideograms, due to the misconception that characters represented ideas directly, whereas some people assert that they do so only through association with the spoken word. Traditional classification Traditional Chinese lexicography divided characters into six categories (). This classification is known from Xu Shen's second century dictionary Shuowen Jiezi, but did not originate there. The phrase first appeared in the Rites of Zhou, though it may not have originally referred to methods of creating characters. When Liu Xin (d. 23 CE) edited the Rites, he glossed the term with a list of six types without examples. Slightly different lists of six types are given in the Book of Han of the first century CE, and by Zheng Zhong quoted by Zheng Xuan in his first-century commentary on the Rites of Zhou. Xu Shen illustrated each of Liu's six types with a pair of characters in the postface to the Shuowen Jiezi. The traditional classification is still taught but is no longer the focus of modern lexicographic practice. Some categories are not clearly defined, nor are they mutually exclusive: the first four refer to structural composition, while the last two refer to usage. For this reason, some modern scholars view them as six principles of character formation rather than six types of characters. The earliest significant, extant corpus of Chinese characters is found on turtle shells and the bones of livestock, chiefly the scapula of oxen, for use in pyromancy, a form of divination. These ancient characters are called oracle bone script. Roughly a quarter of these characters are pictograms while the rest are either phono-semantic compounds or compound ideograms. Despite millennia of change in shape, usage and meaning, a few of these characters remain recognizable to the modern reader of Chinese. At present, more than 90% of Chinese characters are phono-semantic compounds,