許慎
Han-dynasty scholar Xu Shen in his 2nd-century dictionary "Shuowen Jiezi" classified Chinese characters into six categories ( "liùshū", Japanese: "rikusho"). The traditional classification is still taught but is problematic and no longer the focus of modern lexicographic practice, as 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.