A tetrode is a vacuum tube having four active electrodes. There are several varieties of tetrode with screen-grid tube, and the beam tetrode being the most common. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.