There are four types of cavities of the lumbosacral joints. Type A: The synovial membrane is all around attached to the margin of the articular facet of the superior articular process of the sacrum. Type B: The synovial membrane extends to the posterior surface of the sacrum forming a recess at the root of the superior articular process; this recess communicates widely with the cavity of the joint. Type C: The synovial membrane also forms a recess similar to that of type B, but the gap of communication is narrowed by a fibro-adipose meniscoid. Type D: The synovial membrane is attached in the same way as described in type A; besides a synovial bursa non communicating with the cavity is found at the root of the superior articular process. We have found 43% showing type A, 33% type B, 16% type C and 8% type D. The recesses and bursae described above enable the inferior articular process of the fifth lumbar vertebra to slide at the posterior surface of the sacrum, thus avoiding a painful rubbing during dorsiflexion of the lumbar spine. The existence of these sliding facilities does not depend on the range of the lumbosacral angle nor on the quality of the lumbosacral intervertebral disc.