The height adjustment of the back braces serves the twofold purpose of tuning the frequencies of its main modes compared to the soundboard resonances, and of adjusting the amplitudes and the shape of the modal areas. The target is to reach an optimum coupling top to back via the air in the cavity. As this chapter shows, the process is done firstly with the back on the mould, both looking to the Chladni patters obtained via the electromagnetic exciter (to check the regularity of the vibrating areas), and using the acoustic hammer (to obtain optimal amplitudes and shapes of the modes). Once the result from the mould is appropriate, the back is fixed to sides, at first provisionally and then definitely. The impact of adding a fourth brace is discussed as well.