some of these characters are also used in china; others are used only in vietnam. chinese characters were introduced to vietnam when the han empire invaded the country in 111 bc. even after vietnam became independent in ad 939, the country continued to use classical chinese () for official purposes. in the 1920s, vietnam shifted from traditional characters to the latin alphabet. the han-nom institute was founded in hanoi in 1970 to collect and study documents written in the traditional script. the institute has submitted a list of 19,981 sino-vietnamese characters to unicode for electronic encoding. this includes a core set of 9,299 characters called the nôm ideographs.