A MIDI control available on some electronic musical instrument keyboards where, having depressed a key to play a note, the application of an additional firm pressure controls an effect such as vibrato (fundamental-frequency modulation) or tremolo (amplitude modulation) on the sound. Aftertouch is either monophonic, where the effect is applied to all notes currently depressed, or polyphonic, where it is applied individually only to each note that is depressed more firmly.