Body linens

That is to say - shirts and braies for the gents, smocks for the ladies. Cotton was a rare expensive luxury fabric in the later Middle Ages, as was silk. Nearly everyone would have worn linen next to the skin. Body linens were frequently washed and would have softened with use so are actually very comfortable and cool (a major factor if you are wearing armour on a hot day). For men a set of body linens consists of braies (like boxer shorts with a drawstring waist) and a shirt, which unlike later shirts would be simple in form without a collar or frills anywhere (I'm talking of the majority here; some aristocratic dandies may have had something fancier - there is a self-portrait of Durer wearing an embroidered shirt for instance, but even this has a simple boat-neck). Some had a slit at the front for ease of getting the shirt on and off, some of these have a tape stitched around the collar for tying. Earlier braies were much fuller and longer, with a drawstring (or tape) which emerged from the waist at several places to provide an anchor-point for the laces holding up the separate hose. Once joined hose with a codpiece (see separate entry on hose, to follow) started to be worn from around 1400 onwards, braies 'shrank' to something a lot briefer and closer-fitting.

Any shirt longer than hip length becomes a nuisance under joined hose and leg armour; the few contemporary depictions of shirts bear this out. Old linens, btw, would be recycled as linen-rag paper.

A shirt with a belt over the top is fine if you want to play at being a pirate but nonsensical as period costume (what's holding your hose up? And if you're wearing split hose tied to the braies-girdle, why are you showing everyone your knickers?).

The above picture is from the Tres Riches Heures du Duc de Berry of around 1412-16 (French); the male reapers in the background have stripped down to shirts and braies; the women in front are revealing the sleeves and - in the case of the right-hand figure - hem of their smocks. The men's shirts represent, unusually, a range of colours, presumably achieved with the standard woad (blue) and madder (read) dyes.

A study of hanged men by Pisanello (Italian), dating from the 1430s. Shows shirts and braies.