Windows are critical to a building’s design and historic character. According to the National Park Service’s (NPS’s) Illustrated Guidelines for Rehabilitated Historic Buildings, “As one of the few parts of a building serving as both an interior and exterior feature, windows are nearly always an important part of the historic character of a building. In most buildings, windows also comprise a considerable amount of the historic fabric of the wall plane and thus are deserving of special consideration in a rehabilitation project.” Given this position, it is not surprising that the agency devotes 22 Preservation Tech Notes to the topic.