backbone vs tail bone

backbone and tail bone both are nouns.

Word	Noun	Adjective	Verb	Adverb
backbone	Yes	No	No	No
tail bone	Yes	No	No	No

backbone (noun)	tail bone (noun)
the part of a network that connects other networks together	the end of the vertebral column in humans and tailless apes
the part of a book's cover that encloses the inner side of the book's pages and that faces outward when the book is shelved
fortitude and determination
the series of vertebrae forming the axis of the skeleton and protecting the spinal cord
a central cohesive source of support and stability