The human coccyx, commonly called the tailbone, is made up of three to five small vertebrae, with four being the most common number. Unlike the vertebrae higher up in your spine, these tiny bones are mostly fused together into a single triangular structure at the very bottom of your spinal column.
Why the Number Varies
Not everyone has the same number of coccygeal vertebrae. A computed tomography study published in Folia Morphologica found that about half of people (49.5%) have four coccygeal vertebrae, while roughly 36% have five, 11% have three, and just under 3% have only two. This variation is completely normal and has no effect on how well the tailbone functions. Most anatomy textbooks list four as the standard count, but your actual number could be different without you ever knowing.
These segments are much smaller than the vertebrae in the rest of your spine. They lack the features you’d see in a typical vertebra, like a spinal canal or clearly defined processes. In many adults, some or all of the coccygeal segments fuse together over time, making them harder to distinguish as separate bones on an X-ray.
How the Coccyx Develops
At birth, the coccygeal vertebrae are mostly cartilage rather than solid bone. They gradually harden and fuse in a predictable pattern from childhood through early adulthood. Research using MRI and CT imaging has shown that the ossification and fusion process continues from the fetal period all the way up to around age 30. This is one reason the number of visible segments can differ depending on a person’s age and when imaging is done. In younger people, the segments are more clearly separated, while in older adults they may appear as a single fused mass.
What the Tailbone Actually Does
The coccyx is sometimes described as a vestigial tail, but it plays several active roles in the body. It serves as an anchor point for the gluteus maximus (the large muscles in your buttocks), the levator ani (a key pelvic floor muscle), and the muscles of the anus. It also supports tendons and ligaments that connect to surrounding structures in the pelvis.
When you sit down, especially when leaning back, your body weight shifts onto the coccyx along with the two sit bones of the pelvis. The tailbone acts as a tripod leg, helping to stabilize you. It also provides structural support to the pelvic floor, the hammock of muscles that holds up the bladder, intestines, and (in women) the uterus.
Differences Between Men and Women
The coccyx is shaped and angled differently depending on sex. In women, the tailbone tends to be straighter, while in men it curves more sharply inward. One study in Clinics in Orthopedic Surgery measured the angle between coccygeal segments and found an average of about 45 degrees in women compared to 54 degrees in men. The female coccyx is also generally more mobile, which is thought to help accommodate childbirth. These structural differences are one reason women experience tailbone pain (coccydynia) four to five times more often than men.
When the Tailbone Causes Problems
Tailbone pain, or coccydynia, most often results from a direct impact like a fall onto a hard surface. The small coccygeal vertebrae can bruise or fracture, and the joints between unfused segments can become inflamed. Prolonged sitting on hard or narrow surfaces, cycling, and childbirth are also common triggers. The pain typically worsens when sitting, leaning back, or standing up from a seated position.
Because the number and fusion pattern of coccygeal vertebrae varies so much from person to person, imaging results can sometimes be tricky to interpret. What looks like a fracture line might actually be a normal joint between unfused segments, and what appears to be a single bone might be several fully fused vertebrae. This natural variation is one reason a clinical exam, not just an X-ray, is important for diagnosing tailbone injuries.