Antique civilization wasn't familiar with underwear, but with the expansion of religion the human body was declared sinful and church demanded that people fully cover it. So the history of medieval underwear began, that for a long time involved only a shirt, named ‘chemise’.
Chemise styles were the same for all medieval classes, both laity and clergy, men and women. The main and only difference was the quality of the fabric used. It depended on the owner's status - rough thick flax and sackcloth were used by peasants, fine linen chemise suited a medieval citizen, but wearing silk underwear befit only the nobles and the rich. So when you buy medieval underwear, consider the origin of your character.
Undershirt was worn right next to the skin. It was a modest T-shaped cut, supplemented by two or four gores sewn in at the waist for easy walking. Further volume at the chest was created by the underarm gores. Long rectangular sleeves were narrowed at the elbow or at the wrist with laces. There was no embroidery nor decorations yet, however, each shirt was unique as made according to the size of the future owner - same as we make medieval chemises for sale now in ArmStreet.
Initially, chemise was not intended for extraneous eyes. Its edges had to be hidden by pinning to the neckline of the dress. Still the floor-length hem might be peeking out from under the dress, sweeping the floor and leaving only the shoe toes open. Creating medieval underwear for sale, we take into account the fashion trends of Medieval era - so, in the 12th-century women's chemises became more fitted. The trapezoidal cut was still on top, but now it could be equipped with the side slits. It appears that the vast majority had long sleeves, besides several survived pictures depicting sleeveless chemises with spaghetti straps.
If you’re going to buy medieval chemise of 13-14 centuries, note that they were getting shorter, and typical length of that time was just above the ankle. Men's chemise was flared and tended to get shorter as well, usually, it was about mid-thigh length. The sleeves were long and narrowed. Neckline tended to boat shape, sometimes combined with a vertical slit. In the late 14th century it was accommodated with the collar ties.
Subsequently, the chemise was replaced with underwear separates because of the fashion for tight button-up clothing. Nevertheless, it can surely be considered as a prototype of the modern shirts, slips, and nightgowns.