A tetrode is a type of vacuum tube that contains four active elements: a cathode, an anode, and two control grids. It was developed to improve the performance of earlier vacuum tubes, such as the triode, by providing better amplification and reduced distortion. Tetrodes are commonly used in radio frequency applications and audio amplification. The two grids in a tetrode allow for better control of the electron flow between the cathode and anode. This design enhances the tube's efficiency and stability, making it suitable for various electronic devices, including radios and amplifiers. Tetrodes played a significant role in the development of early electronic technology.