The first devices designed to change frequency in the manner described above seem to have been developed by the French, who simply put two grids into what would otherwise have been an ordinary triode valve (the bi-grille or bi-grid). Although technically a four electrode device, neither the term tetrode nor the tetrode valve as it is known today had yet appeared. The bi-grid differed from the later tetrode because the second (outer) grid was coarsely wound compared with the tetrode's screen grid which had to be finely wound to provide its screening effect.[1] Each grid was able to accept one of the incoming signals, and the non-linearity of the device produced the sum and difference frequencies.