pelvis
the sacrum is part of the vertebral column. it is made up of five bones that fuse together over time
The Coccyx bone, which is a very small triangular bone forming the end part of the spine. It is actually 4 separate bones which are fused into one.
The spine (specifically L5, the 5th lumbar vertebra). The pelvis also attaches to the sacrum but I think that joint is fused.
The spine has to end somewhere, and that last vertebra is called the coccyx. If it didn't end there, we would have tails.
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