Fashion in the Age of Datini

Breeches

Men (and probably some women, as astridaschaeffer argues) wore linen or hemp shorts for modesty and sanitation (undyed linen and hemp were the only fabrics which could be washed in hot soapy water without danger of fading or shrinking). These are commonly worn by men in paintings and literature, and occasionally show up in inventories and descriptions of merchandise produced by different guilds: rich men died owning up to six pairs for their own use (Pisetzky vol. 2 pp. 22, 23, Brandi p. 133, Datini inventory). They were held up by a belt in a casing (which could also support the hose), a drawstring, or later by knotting them closed at the hip like a modern bikini bottom. Texts describe the ideal breeches as white linen, and most artwork supports this, but a few breeches in 15th century paintings are dark blue or black instead. There are some articles on the subject at https://bokeofthewardrobe.wordpress.com/

During the 14th century these were called breeches in English, mutande in Italian, and braes in French: the 'proper' Latin name was braccae.

To the best of my knowledge, no breeches survive from the fourteenth century, and artwork rarely shows seams except at the sides of the legs. Styles in artwork are quite diverse, and presumably many wearers (and makers) had their favourite styles. A modern pattern for the style in the following paintings is available in Singman and Maclean's Daily Life in Chaucer's England.

A man in a breech (Fr. braes) in a 14th century fresco from the left transept of Santa Anastasia, Verona. I suspect that it was painted after 1360 given the shape of his chest and style of his hair and beard.
Detail of the breeches worn by St. Sebastian in the Hours of Catherine of Cleves (Utrecht, circa 1440; Morgan Library, New York, MS M.917/945, pp. 252/3 http://www.themorgan.org/collection/hours-of-catherine-of-cleves/319

Robert Macpherson is working on a typology of breeches in art from the 13th century to the beginning of the 16th: type I ("Maciejowski"), type II, type III, and type V are most relevant for trecento Italy. A series of reconstructions are posted on a discussion forum.