Evidence of Korean Dangui came from China

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Tang dynasty standard formal clothes of women is wearing Big sleeves Qunrui(大袖裙襦) . And Beizi(背子) was worn outside the Big sleeves Qunrui(大袖裙襦) ..... So based on the analysis above, the so called long Beizi in Tang dynasty standard is a kind of clothes which is long sleeves, two裾(Ju) covering each other(Notes of translator:裾指衣服的大襟，<说文通训定声>载“裾，衣之前襟也。今俗曰大襟。”兩裾相掩说明这种服饰是交领类的，交领的正式名称也叫直领大襟，two裾(Ju) covering each other means this kind of clothes is cross collar交领) and axillary in clothes’ two side not suturing. And it was almost the same to Long Ru(长襦) worn by men. And Beizi which is shorter is called short Beizi, which is called Dangui nowadays……..So in short, in today tradition, Dangui, Wonsam, 대삼（大衫）【Notes in blue clollar:Today 상복（丧服） mainly use big sleeves】,장삼(長衫) and 배자(背子) are both not too far from Chinese standard.

宋文欽, 閒靜堂集 卷之七, 雜著 婦人服飾 衣,

唐制命婦服。大袖裙襦爲禮衣。又云大袖在背子下。..... 此數語參互考之。其所謂長背子。引爲長袖。兩裾相掩。兩腋不縫者。與今丈夫長襦略同。稍短之則當爲短背子。卽今之唐衣也.......以此觀之。則今俗所用唐衣圓衫大衫(今世喪服。多用大袖)長衣背子之類。皆與中華舊制不遠。