The space required for reset.
On an upright, this space is found between the jack and the hammer heel:
(insert pic from JENN)
On a grand, this space is found in three places:
Between the jack and the knuckle:
Between the bottom of the key and the front rail punching:
At the second rise of the hammer after checking:
If there is no space in any one of those places, then there is no aftertouch. Correct aftertouch represents the sum total of correct regulation specs involving key height, blow distance, dip, lost motion, checking, letoff, and drop.