In this paper a complete theoretical solution is given for a bar vibrating in flexure taking account of rotary and lateral inertia. The solution shows that the frequency of a bar free to vibrate on both ends, is asymptotic to the frequency given by the usual solution, neglecting rotary inertia, when the ratio of width to length is small, and approaches the frequency of a bar in longitudinal vibration when the width becomes comparable to the length. The theoretical frequencies have been compared with the published results of Harrison on the frequency of a quartz crystal vibrating in flexure, and have been found to agree within one percent for a crystal whose width is less than half its length.