The term six types of Chinese characters (liushu 六書) has been invented by the Later Han-period 後漢 (25-220 CE) scholar Xu Shen 許慎 (c. 58-c. 147), compiler of the famous dictionary Shuowen jiezi 說文解字. Although Xu shortly explains these styles in the postface (xu 叙) of his dictionary, he does not consistently make use of this concept.
Zheng Qiao tried to develop a more concrete concept and increased the six styles to twelve by considering various combinations of symbol and sound or symbol and meaning for the creation of characters. Zheng's attempts laid the ground for numerous later studies on this issue, like Dai Tong's 戴侗 (1200-1285) Liushugu 六書故, Yang Huan's 楊桓 (1234-1299) Liushutong 六書統, Zhou Boqi's 周伯琦 (1298-1369) Liushu zheng'e 六書正訛, Zhao Guze's 趙古則 (1351-1395) Liushu benyi 六書本義, Wei Xiao's 魏校 (1483-1543) Liushu jingyun 六書精蘊, and Zhao Huanguang's 趙宦光 (1559-1625) Liushu zhangjian 六書長箋. Zheng Qiao's own essay Liushuzheng 六書證 is lost.
Zheng Qiao tried to show that the character types xiangxing 象形 "illustration of a shape" and zhishi 指事 "pointing at things" actually followed the same idea of drawing something, a fact also valid for the types xiesheng 諧聲 "harmony and sound" and zhuanzhu 轉注 "comment by turning" which are both relying on the sound of the word.
Of all character types, Zheng Qiao says, drawn objects (xiangxing) are the most basic. What can not be drawn as an object, is an affair (shi 事). An affair that can not be pointed at (zhishi) with a written character, is an idea (yi 意). Ideas that cannot be brought together (huiyi 會意 "assembled meanings") in one written character, have to be expressed by a sound (sheng 聲). Sounds have all certain rules of harmony (xie 諧). Only if these means of writing are not sufficient, it is necessary to borrow (jiajie 假借 "wrongly borrowed") another character for a word. This is Zheng Qiao's explanation of the development of the six types of characters.