Xu Shen
The derivative cognate () is the smallest category and also the least understood. In the postface to the "Shuowen Jiezi", Xu Shen gave as an example the characters "kǎo" "to verify" and "lǎo" "old", which had similar Old Chinese pronunciations (*khuʔ and *C-ruʔ respectively) and may have had the same etymological root, meaning "elderly person", but became lexicalized into two separate words. The term does not appear in the body of the dictionary, and may have been included in the postface out of deference to Liu Xin. It is often omitted from modern systems.