The human spine has 33 vertebrae, stacked into five distinct regions from the base of your skull to your tailbone. Of those 33, only the top 24 remain separate, movable bones in adults. The remaining nine fuse together during development to form two solid structures at the base of the spine.
The Five Regions of the Spine
Each region of the spine has a different number of vertebrae, and each is shaped for a specific job.
- Cervical (neck): 7 vertebrae, labeled C1 through C7
- Thoracic (mid-back): 12 vertebrae, labeled T1 through T12
- Lumbar (lower back): 5 vertebrae, labeled L1 through L5
- Sacrum: 5 vertebrae, fused into one bone
- Coccyx (tailbone): 4 vertebrae, fused into one small bone
That adds up to 33 total: 7 + 12 + 5 + 5 + 4. The sacral and coccygeal vertebrae fuse gradually from the fetal period through about age 30, which is why some sources describe the adult spine as having 26 bones (24 individual vertebrae plus the sacrum and coccyx).
What Each Region Does
The cervical vertebrae are the smallest and most delicate in the spine. They have a large spinal canal because the spinal cord is thickest at this level, carrying signals to the entire body below. The top two cervical vertebrae are unique: C1 (the atlas) lets you nod your head up and down, and C2 (the axis) lets you rotate your head side to side. The remaining five cervical vertebrae allow bending and tilting of the neck but very little rotation on their own.
The 12 thoracic vertebrae each connect to a pair of ribs, forming the bony cage that protects your heart and lungs. This rib attachment is also what makes the mid-back the least flexible part of the spine. Rotation is possible here but limited, and forward bending is restricted compared to other regions.
The five lumbar vertebrae are the largest and strongest in the spine. They carry the weight of everything above them, which is why they have thick, kidney-shaped bodies. The lumbar spine allows significant forward and backward bending and some side-to-side motion, but essentially no rotation. This combination of heavy load and high mobility is why the lower back is the most common site of spinal pain and disc problems.
The sacrum, a triangular bone formed from five fused vertebrae, sits between the two hip bones. It transfers the weight of your upper body to your pelvis and legs, not through the joints themselves but through a set of powerful ligaments connecting the sacrum to the hip bones. Below the sacrum, the coccyx is a small remnant of four fused vertebrae that serves as an attachment point for pelvic floor muscles and ligaments.
Not Everyone Has Exactly 33
The textbook count of 33 is the most common pattern, but it’s not universal. Somewhere between 3% and 36% of people have what’s called a lumbosacral transitional vertebra, meaning the boundary between the lumbar spine and sacrum isn’t quite standard. One large study found this variation in about 19% of patients. In most cases, the lowest lumbar vertebra (L5) partially or fully fuses to the sacrum, a pattern called sacralization. Less commonly, the top sacral segment separates and behaves more like a lumbar vertebra, called lumbarization. These variations give someone what appears to be four lumbar vertebrae and six sacral, or six lumbar and four sacral, depending on the direction of the shift.
Most people with these variations never know they have them. The vertebrae still function normally, and the variation itself doesn’t reliably cause pain. The more practical concern is medical: if a surgeon or radiologist counts vertebrae from the bottom of the spine up, a transitional vertebra can throw off the numbering and lead to the wrong level being identified on imaging. Wrong-level spine surgery happens in an estimated 0.1% to 2.1% of cases, and transitional vertebrae are a recognized contributor. This is one reason spinal imaging reports often note whether a transitional vertebra is present.
Why the Vertebrae Get Bigger as You Go Down
There’s a simple mechanical logic to spinal anatomy. Each vertebra supports the weight of everything above it, so the bones get progressively larger and sturdier from neck to lower back. A cervical vertebra might be roughly the size of a large coin, while a lumbar vertebra is closer to the size of your fist. The spinal canal follows the opposite pattern, starting wide in the neck (where the spinal cord is thickest) and narrowing through the lumbar region, where the cord has already branched into individual nerve roots.
The shape of each vertebra also determines what movements that part of the spine allows. In the thoracic region, the joint surfaces are angled along a curve that permits small rotational movements. In the lumbar region, the joint surfaces face each other in a front-to-back orientation, which is ideal for bending forward and backward but physically blocks rotation. These structural details explain why twisting motions come from the mid-back and hips rather than the lower back, even though many people assume otherwise.