These are the lowest layer of clothing worn by people of this century. They are large, baggy drawers made from linen and they seem to be worn by men from all classes of society under their normal clothing. We know what they look like from well-documented illumination examples of hot field-workers dispensing with all their clothes other than their braies for both modesty and coolness.
The look can be recreated with approximately 100" of 30"-wide fine linen. Simply make a pair of shorts far wider than you can possibly imagine, and incorporate a roll at the top to contain a drawstring. The excess material (almost three times the actual waist line) is bundled up all around the body. The drawstring roll requires two slits for attaching the hose (leggings) at the front.
The inside of each leg is slit virtually up to the crotch so that the mass of material here can be overlapped neatly under the hose without creasing. This slit also enables gentlemen "normal functionality" whilst standing. When braies are worn without hose, the front corner of the leg cloth is often shown wrapped around behind the leg and tied up to the hose tie as demonstrated by our knight's right leg. This is a handy way of dealing with all the cloth involved.
In use, the braies are suprisingly cool and comfortable, if not a little "airy" and (ahem!) "freer" than underwear that we've got used to in the 20th Century. Putting them on takes a couple of minutes, and the only drawback I've found is whilst sleeping in them - the material tends to lag as you roll over during the night. The rolled up waistband is deeply unattractive to modern eyes, but is an almost essential item when chausses are being worn.
Click on braies in the menu bar to see how to make a pair.