The weight of the head and trunk is transferred to the lower extremities through that segment of the spine articulating either directly or indirectly with the ilii. When a lumbosacral transitional vertebra is present the position of the weight-bearing platform usually changes. A bony entity is formed delineated superiorly by the top of the transitional vertebra and inferiorly by the lower level of the sacroiliac joints. This segment is designated as the transitional lumbosacral osseous complex. Measurements of its length in the three phylogenetic categories in man, including measurements of the articulating portion of the sacrum in the human mode, showed a range of 68 mm to 157 mm and variations from 2.0 to 3.7 in vertebral lengths.