Aftertouch is a feature on keyboards and synthesizers that generates a control signal based on the pressure applied to a key that is already pressed.

Think of Aftertouch as a "second stage" of expression on the key. First, you press it to play the note. Then, by applying more force and pressing it deeper, you activate Aftertouch, which modulates some aspect of the sound in real time.

Most keyboards implement Channel Aftertouch (or Mono Aftertouch), which uses a single sensor strip under all the keys. It detects the overall pressure applied to any key and sends a single control signal that affects all notes that are sounding. A rarer and more expensive version, the Polyphonic Aftertouch, has individual sensors for each key, allowing you to modulate notes independently within a chord.

Aftertouch is commonly used to add expression, controlling parameters such as:

• Vibrato: Increase the intensity of the vibrato in a string or synth lead sound.

• Filter: Open the filter to make the sound brighter.

• Volume: Increase the volume of a note to give emphasis (swell effect).