tail bone vs backbone

tail bone and backbone both are nouns.

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

tail bone (noun)	backbone (noun)
the end of the vertebral column in humans and tailless apes	the part of a network that connects other networks together
	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