Traditional glass used to glaze windows is an often-overlooked element of traditional buildings yet contributes much to their character and appearance. The imperfections in the glass produced before the 1950’s create a characteristic and appealing shimmering effect on the exterior by reflecting light at many angles in a way that modern glass does not. Equally, a slight distortion of the view out of a window is one of the charms of traditional glass. This INFORM is intended to outline the types of traditional plain glass and glazing systems most commonly encountered by building owners, and to give general guidance on their maintenance and repair.