your spine....
A back bone allows an human to stand up straight. - the position or stance taken for a fight. If you say someone "has no back bone" it means they are not willing to stand up for themselves.
carbon is the back bone of organics molecules because it catenate. it's chain is a straight chain.
About as straight as a roundabout
A straight break across the bone is called a transverse fracture.
It keeps them in business. Someone would much rather come back to a clean highway motel, than a dirty Hilton.
Breast bone is round and a leg bone is straight
Inertia
there is a bone between your legs that sometimes gets straight when you sit down.
a trombone?
Your grothplates and bone
a bone keeps us stable so we wont be a body laying on the flore
The gravity from the Sun keeps pulling it back into an orbit - therefore they follow an elliptical path which brings them back towards the sun periodically..