Morse code was widely used in the early days of radio because it was easy both to produce the signal as well as receive it. Because the output from the amplifier does not have to closely match the original modulation of the received signal, in contrast to voice broadcasts, any number of simple amplification systems could be used. One was due to an interesting side-effect of the construction of early triode amplifier tubes. If both the plate (anode) and grid are connected to resonant circuits tuned to the same frequency, stray capacitive coupling between the grid and the plate will cause the amplifier to go into oscillation if the stage gain is much more than unity.