S1 through S5 is a reference to the five (S1, S2, S3, S4, S5) vertebrae (bones that form an opening in which the spinal cord passes) that are near an area of the body known as the sacrum. The sacrum is a large bone in the lower part of the spine that is shaped like a triangle. The letter "S" stands for "sacrum." So S1 refers to the 1st sacral bone, S2 to the 2nd sacral bone, S3 to the 3rd sacral bone, and so on, until S5, which is the last sacral bone. S1 is at the top and S5 is at the bottom, with the numbers in between going down the body in numerical order. For example, S3 is below S2, S4 is below S3, and so on. These numeric levels are often mentioned on imaging studies of the spine such as computerzed axial tomography (CT) scans or magnetic resonance imaging (MRI). CT scanning is an advanced imaging technique that uses x-rays and computer technology to produces clear and detailed picture of the body area being examined. MRI scans produce extremely detailed pictures of the inside of the body by using very powerful magnets and computer technology.
Other designations for other levels of the vertebrae include a) C1 through C7, where C stands for the cervical (neck) area; b) T1 through T12, where T stands for the thoracic (chest) area, and c) L1 through L5, where L stands for the lumbar (lower back) area.