Why was ancient Joseon clothing so open?

At the beginning of the last century, American missionaries used the most advanced photo bleaching technology at that time to take photos of the civilian life in Joseon. At that time, a kind of women's short clothes worn in summer was circulated in Joseon folklore, and it was called the breast-baring outfit for the beauty of showing breasts. It was banned during the Japanese occupation.

Possible reasons:

(1) Traditional customs. As can be seen in the painting, this kind of dress has been practiced for a long time.

(2) To facilitate breastfeeding. As seen in the photo, unmarried women don't wear it, while married women do.