There was a relic in Chartres Cathedral, supposed to have been a chemise worn by the Virgin Mary. By the 13th century, "it was the custom for people to hold pieces of cloth against the relic and have shifts made to measure from the material." The replica garment was expected to develop holiness by contagion. This was recorded in the Cantigas de Santa Maria and other collections of accounts of Marian miracles, which tell of a knight of Aquitaine wearing such a shirt into combat, and being miraculously preserved from harm.
The custom seemed to have persisted at least into the 16th century. The Baron de Bueil was wearing such a shirt when shot at the siege of Milan in 1523. The bullet pierced his other garments but not the shirt, and he made an offering of the shirt, bullet and penetrated outer garments at Chartres in 1524.
Presumably, in any cases where the bullet retained enough force to penetrate the shirt, no offerings were made.
A short document, Modus armandi milites ad torneamentum, in a collection compiled around 1330, Additional MS 46919, recommends that for tournaments a knight wear a camisia de Chartres over his aketoun and beneath his hauberk.
Du Cange's Glossarium quotes an undated manuscript (Probat. Histor. Britan. col. 1222.) that advises that when knights fight duels, both should wear a chemise de Chartes between their armors: inter armaturas, quas vestire debebant duo milites duello pugnaturi tom. and Roger Mortimer owned an aketon cum una camisia de Chartres in 1322.*
*BL MS Additional 46919 fo. 87r; Mortimer Inventory, p.361.