What does the "K" stand for in CMYK?

Ever wondered why black is represented with a 'K' in the CMYK colour break down????

CMYK stand for Cyan, Magenta, Yellow and Black, the colours used in four colour process printing. So this begs the question - why is it not called CMYB?
Well the answer is for two reasons:

1. Back in the old days of printing when all printing was done using plates the black plate was know as the key plate. This was because it contained the artistic detail or 'key' information.

2. To avoid any confusion with the RGB colour process which stands for Red, Green, Blue.