Clearing the cervical spine after blunt trauma remains a challenge. Even in the computed tomography era many emergency departments worldwide still use classical X-rays in first evaluation. Low odontoid fractures are frequently missed, especially in unconscious patients where an open mouth view is not available. Evaluation of the Harris ring in the lateral view can improve identification rate. We studied the diagnostic values of this sign and the educational effect on trainees. Lateral views of 12 computed tomography confirmed low-axis fractures and 13 controls were randomly presented to 17 residents (traumatology, neurosurgery and emergency medicine) and five experienced radiologists. After the residents were taught the use of the axis ring, they had to review the set. Diagnosis was scored with a degree of certitude from 5 to 1. The specificity and sensitivity for the radiologists was 88% (confidence interval 80–96) and 82% (confidence interval 72–91), respectively. The effect of the education on the scores of the residents was evaluated using the Wilcoxon ranking test with a significant effect for the traumatologists (P=0.0008), emergency physicians (P=0.0005) as well for the neurosurgeons (P=0.0087). The axis ring can be a useful diagnostic tool in identifying low odontoid fractures on cross-table cervical spine X-rays. It is easy to teach and should be included in X-ray-based C-spine clearing protocols.