Mechanical resonators for making timepieces have the drawback of not being simultaneously efficient, compact, and inexpensive. The invention is a planar structure of a mechanical resonator (100) that is suitable for reducing bulkiness and manufacturing cost and comprises a bar (R) vibrating in longitudinal expansion/compression resonance and two beams, (12) each vibrating simultaneously in longitudinal expansion/compression and it bending oscillation in the plane of the structure and connected to the bar by a connection element (11) on the median plane (π) of the bar, thereby enabling the effects of the transverse deformations of the bar, due to the Poisson ratio of the material, to not be transmitted to the attachment pads (PF) of the resonator. Thus, the quality factor of the resonator can be very high.