The Centre International du Vitrail occupies a 13th-century cellar in the shadow of the cathedral and is dedicated entirely to the art of stained glass — appropriate given that Chartres holds the finest collection of medieval glass in the world. The permanent display explains the history and technique of glass-making from ancient times to the present; temporary exhibitions bring in contemporary artists working in the medium.
The cellar space itself is atmospheric, with vaulted Romanesque architecture setting off the luminous glass on display. There is also an active workshop where traditional techniques are practised and taught. Even if the exhibitions feel modest compared to the cathedral itself, the context they provide for understanding what you are looking at in the windows above is genuinely useful.