chinese characters were introduced to vietnam after the han empire conquered the country in 111 bc. independence was achieved in 939, but the chinese writing system was adopted for official purposes in 1010. vietnamese began to use chinese characters to write in their own language at about this time. the van ban bell, engraved in 1076, is the earliest known example of a nôm inscription. nguyen thuyen composed nôm poetry in the 13th century. however, none of his work has survived. the oldest surviving nôm text is the collected poetry of king tran nhan tong, written in the 13th century. many nôm documents were destroyed by the chinese during the ming occupation of 1407-1428. nguyen trai (1380–1442) wrote both chinese and nôm literature in the 15th century. trinh thi ngoc truc, consort of king le than tong, is credited with a 24,000-character bilingual chinese-to-vietnamese dictionary written in the 17th century.