It is very rare that a single joint occurs by itself, or that multiple joints have no recognized mutually common attitudes. In fact, joints commonly have consistent orientation and they are pervasive due to the distributed nature of fracturing. This raises the concept of a joint set, which consists of a collection of parallel or sub-parallel individual joints or echelon joint zones.

We selected some joint sets from sedimentary and igneous rocks at different scales in Figures 1 to 4. Figure 1 shows a set of fins defined by a set of deeply eroded, slightly sheared joints or joint zones in the Slickrock member of the Entrada Sandstone outcropping in the northern limb of Salt Valley Anticline at Arches National Park, Utah. Figure 2 shows the same joint set responsible for the fins in the previous figure, this time on the top surface of the Moab member which is stratigraphically above the red colored Slickrock member of the Entrada Sandstone shown in the previous image, here without much erosion along the joints or joint zones.

Figure 1. A set of joints or joint zones dividing the Entrada Sandstone into a series of fins at the northern limb of Salt Valley Anticline, Arches National Park, Utah.

Figure 2. Bird's eye view of a systematic joint set in the Moab member of the Entrada Sandstone exposed on the northern limb of the Salt Valley Anticline, Arches National Park, UT. The width of the photo taken from an airplane is estimated to be about 300 meters.

Figure 3 and Figure 4 show two examples of joint sets on glaciated surfaces of granitic rocks of the Sierra Nevada, Nevada and California. Figure 3 is a photograph of a joint system at Donner Pass, at the Nevada-California border and Figure 4 is map view of a joint/vein array at Bear Creek, California (Segall and Pollard, 1983). Note that no shear offset is visible across an older aplite dike (diagonal in the view).

Figure 3. Outcrop photo showing a joint set exposed on the glaciated surface of granitic rocks at Donner Pass, Sierra Nevada, California. Note that most joints are heavily eroded and some have splays indicating small shear offset across them after the formation of the joint set. Another interesting property of the individual joints of the set is that they have different lengths in the view area. Men for scale.

Figure 4. Map showing a set of joints and veins in granitic rocks of the Sierra Nevada, California, near Ward Lake. The diagonal thick line, which is the trace of an older aplite dike, shows no detectable offset at its intersection with the joints/veins even though minute shear separation has been measured across some of the joints in the map area and in its surroundings. From Segall and Pollard (1983).

Three of the most important geometric properties of a joint set are length, height, and spacing distributions, which evolve as a joint set develops. These properties are affected by many factors including the magnitude and rate of strain, layering and layer interfaces, layer thickness, and elastic or plastic properties of rock. See 'Geostatistical Properties of Joint Sets' for additional information.