Regional joints in southern Alberta form patterns that persist over an area extending from the Rocky Mountain Foothills to the Saskatchewan border. These patterns persist vertically through a section of rocks ranging in age from Late Cretaceous to Late Paleocene.The basic unit of jointing is an orthogonal system consisting of two sets of extension fractures. Two or more orthogonal systems may be present at a given locality creating a complex pattern of joints. System I predominates and has sets trending approximately 65 °and 155°, or roughly normal and parallel to the Rocky Mountains. System II joints trend approximately 5 °and 95°, but swing about 15 °clockwise in the Drumheller area. A system having sets trending 45 °and 135 °is present near Medicine Hat.System I joints roughly parallel intermediate width (32-64 km) subsurface structural undulations described by Robinson et al. (1969). System II joints trend parallel and normal to the crest of the Sweet-grass Arch. Further study is needed to determine the age and origin of jointing.Regional joints in southern Alberta show similarities with regional joints in similar structural settings on the Appalachian Plateau and on the Central Oklahoma Plains. Within these areas orthogonal systems of regional joints trend normal and parallel to the adjacent fold belt over vast areas and through great thicknesses of sedimentary rock.