The term 'back bone' refers to the spine, which is made up of several bones. The sacrum is a triangular bone located at the base of the spine, consisting of five fused vertebrae. It is six letters long. The oste is a bone found in the arm, extending from the elbow to the wrist. It is four letters long. The ULNA is another bone found in the arm, located on the medial side of the forearm. It is also four letters long. Together, these three bones make up the back bone, or spine.