What does it mean that X-ray cervical radiographs emphasize long axis parallelism before and after standing?

The long axis is parallel to that of the box.

Align the middle point of the mouth with the instrument panel so that the center line is parallel to the occlusal surface of the maxillary door. If the patient's neck is stiff and can't recline, the X-ray tube can be tilted to the head side, which is called that the long axis is parallel to the long axis of the cassette.

Supine photography table or standing position, with both upper limbs beside you, and the middle of the head facing the center line of the photography table and perpendicular to it. The occipital protuberance is placed 2CM above the center of the IP plate, with the head tilted back, and the necks on both sides can be fixed with the head or sandbags. The patient opens his mouth as much as possible or puts foam in his mouth to bite between the upper and lower teeth for support.