We have introduced Banbi (半臂) before, which is a kind of hanfu very suitable for wearing in summer. The tan collar introduced today is a small classification of Banbi. You will find that the shape of the tan collar is very similar to the collar of the T-shirt we usually wear.
Tan collar (坦领), also known as U-collar, is a kind of short coat with a wide collar. Some of them cover it outside the skirt, some like today's short windbreaker. The lower body is matched with a long dress, which fully reflects the graceful posture and natural beauty of women in the Tang Dynasty. In the Tang Dynasty, the tan collar was more popular, not only for men and women but also for court uniforms.
I spelled out a long picture to introduce it. It's easier to understand：
Because of the wide popularity of Hu culture among different classes in the Tang Dynasty, it became a fashion hanfu for women to wear small sleeves in the Tang Dynasty. Popular in the Sui and Tang courts, and later spread to the folk, enduring.
There are many conjectures about the origin of the tan collar. One conjecture is that the tan collar is developed from the round-necked robe. Another kind of conjecture developed from the vertical collar. It is also speculated that the flat collar is a kind of short coat developed from the top since the Wei and Jin Dynasties.