This article describes an iliac prominence and a sacral cavity, located extracapsularly approximately 15 mm dorsal to the angle created by the auricular facies of the sacroiliac joint. These bony structures are often covered by cartilage on one side, usually on the iliac prominence. The sacral cavity is occupied by areolar tissue, rich in adipocytes. They interface very closely, forming an extracapsular junction, which we have named the "axial sacroiliac joint". The study comprised 27 anatomical specimens, 3 of which were found to be ankylosed. Among the remaining 24, and iliac prominence was demonstrated on 22, and a sacral cavity on all. Descriptions and measurements of the sacroiliac joint, the "axial sacroiliac joint", and the spatial relationships are presented, the sacroiliac joint being the joint of reference. The study includes microscopy of the "axial sacroiliac joint".