A keystone is a curious thing. It has these properties:
It is the last stone to be placed in an arch.
It only becomes a standalone arch once that stone is in place.
It is the stone with the least amount of tension on it in the constructed arch.
Keystone habits have the same properties:
A keystone habit can only be built on solid cornerstone habits.
Once the keystone habit is in place, it forms a structure to elevate with.
The near-term doesn't put tension on a keystone habit - it is the future that wants it.
Cultivating keystone habits for the future, while maintaining cornerstone habits is the ultimate keystone habit.