a utility to configure modifier keys to act as other keys when pressed and released on their own
From Github ReadMe:
xcape allows you to use a modifier key as another key when pressed and released on its own. Note that it is slightly slower than pressing the original key, because the pressed event does not occur until the key is released.
The default behaviour is to generate the Escape key when Left Control is pressed and released on its own.
All modifier keys can be assigned useful second purposes. In conjunction with xmodmap several more modifiers can be created and leveraged with xcape to have a keyboard with even more mapping and short-cutting possibilities.