Final answer:
The angle of the ribs is best demonstrated on the AP projection, being the most curved and posteriorly extended part of the rib cage. The AP projection also clearly shows the head of the rib and rib tubercle articulations with the thoracic vertebrae.
Step-by-step explanation:
The portion of the ribs best demonstrated on the AP (anteroposterior) projection is the angle of the ribs, which is the part with the greatest curvature. This region forms the most posterior extent of the thoracic cage and aligns with the medial border of the scapula. It is significant because the angles of the ribs can be used as landmarks in procedures and diagnosis, as well as in correctly counting the ribs in an AP chest x-ray.Furthermore, in AP projection, visibility of the rib articulations is crucial. The head of the rib articulates with the costal facets on the thoracic vertebrae, while the rib tubercle articulates with the facet on the vertebral transverse process. These articulations, especially the ones closer to the sternum, are well depicted in the AP view, providing detailed images used for clinical evaluation.