An electrode component of many vacuum tubes (not present in diodes). The grid acts as a sort of control gate in tubes. An input signal is applied to the grid and as the voltage of the grid is varied by that signal it will attract more or less of the electrons emitted from the cathode, which enables them to pass through to the plate. You can think of it like a water faucet where the input signal is tied to how ‘open’ the faucet is to the flow of water. This is basic amplifier theory: apply a large voltage from a power supply and use a signal to regulate how much of it gets passed on to the next device (like a speaker). Triode tubes, which get their name from having three electrodes, have one grid that operates as described above. Tetrodes have two grids – one that performs as the grid in a triode does (called a control grid or grid no. 1), and another (called a screen grid or grid no. 2) that is used to reduce the capacitance between the control grid and the plate. Too much capacitance of this sort can cause coupling between the input and output circuits in the tube and make an amplifier unstable – adding the screen grid with a positive voltage applied to it creates an electrostatic shield between the control grid and the plate. Pentodes add yet another electrode called the suppressor grid or grid no. 3. The suppressor grid prevents electrons that may have been dislodged from the plate (called secondary emission), due to the bombardment of the plate by other electrons, from returning to the similarly positively charged screen grid. The electrons are diverted back to the plate, increasing the overall efficiency of the tube.