The finite-element analysis (FEA) is used in this work to study the impedance curves and modes of motion at resonance of nonstandard shear plates, thickness poled, and longitudinally excited. An ecological, lead-free, piezoelectric ceramic of ( 1-x )(Bi0.5Na0.5)TiO3- x BaTiO3 with x =0.06 (BNBT6) composition is studied. The FEA modeling is based on the full matrix of the material coefficients. These are obtained from complex impedance measurements on two-thickness poled resonators. A study as a function of the variations of the dimensions of the plate was accomplished ( t = thickness for poling and L and w = lateral dimensions, where w is the distance between electrodes for the electrical excitation). We aimed to a further understanding, and, thus, the ability to control, the coupling of the main shear resonance and the lateral modes. The use of uncoupled shear modes to obtain the material parameters is a key issue for their determination as complex quantities, thus considering all material losses, electromechanical, dielectric, and elastic.