L1 through L5 refers to the five (L1, L2, L3, L4, L5) vertebrae (bones that form an opening in which the spinal cord passes) that are near an area of the body known as the loin. The loin is the part of the back between the lowest pair of ribs of the thorax (chest) and the top of the pelvis. The pelvis is a massive bone made of hip bones on each side and the front, while the back part is made of the sacrum (a triangular bone) and coccyx (a beak shaped bone).

The letter "L" stands for "lumbus," which is Greek for "loin" (see above for definition). So L1 refers to the 1st lumbar bone, L2 to the 2nd lumbar bone, L3 to the 3rd lumbar bone, and so on, until L5, which is the last lumbar bone. L1 is at the top and L5 is at the bottom, with the numbers in between going down the body in numerical order. For example, L3 is below L2, L4 is below L3, and so on.

The lumbar vertebrae are between the lumbar and sacral vertebrae. Sacral refers to the sacrum, which is discussed above.

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