Why is black abbreviated K in CMYK

I have often wondered why black is shortened as K in CMYK, but apparently not enough to look into untill now.

The rest of the lettes make sense, CMY=Cyan, Magenta, Purple, but why K?
K is for key. And key is short for key color or key plate.
In traditional printing when aligning the different color plates, one plate had to be the key plate that you aligned with, and when doing four color printing this would be the black plate. Though when doing two color printing it was not necessarily a black plate that was the key plate, but that is a technicality.