Sacrum and coccyx. Illustration of the sacrum and coccyx at the base of the spine. The sacrum is the triangular fused bone at the base of the spine. It forms as the sacral vertebrae S1 to S5 fuse between the ages of 18 and 30 years. The coccyx (tailbone) is the final section of the spine, consisting of three to five separate or fused coccygeal vertebrae.
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