According to the “Modern Homes Survey” by the National Trust for Historic Preservation,
“A ribbon window is a continuous groove between adjoining planar surfaces. In Modern architecture, reveals were often used at edges of building elements, such as walls and cabinets, to create the illusion that the elements are planes or solid objects floating in space rather than attached to adjacent building elements. Also, the continuous recess between a door or window frame and the surface of the adjacent wall.”
- “Glossary,” Modern Homes Survey: New Canaan, Connecticut, National Trust for Historic Preservation, PDF.