A triode is an electronic amplifying vacuum tube which consists of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Invented in 1906 by Lee De Forest by adding a grid to the Fleming valve, the triode was the first electronic amplification device, and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today their main remaining use is in high power RF amplifiers in radio transmitters and industrial RF heating devices. The word is derived from the Greek triodos, from tri- (three) and hodos (road, way), originally meaning the place where three roads meet.