A vacuum tube is an electronic device consisting of a sealed glass or metal container with two or more electrodes. The most common types are the triode (6 letters), tetrode (7 letters), DIODE (5 letters), dynatron (8 letters), and permatron (9 letters). Triodes, tetrodes, and diodes are all used to control current flow in circuits, while dynatrons and permatrons are used for radio frequency amplification. Vacuum tubes are no longer widely used, having been replaced by transistors and integrated circuits.