CMYK Black Is Not Quite Black

CMYK black, is great for text, allowing for a crisp and sharp reproduction. On large areas of flat black however, 100K doesn't really have much impact as black, more as a dark grey, especially when uncoated.
The solution to this is to add a little extra colour to the mix, this is known as creating a Rich Black. The two most common Rich Blacks are those adding Cyan or Magenta.

Rich Cool Black
Cyan – 40%
Yellow – 0%
Magenta – 0%
Black – 100%

Rich Warm Black
Cyan – 0%
Magenta – 40%
Yellow – 0%
Black – 100%

There are many more variations floating around, including 60% rather than 40% and some using a combination of all four colours.
Rich Black should ideally only be used on blocks of black and large titles as the two coats of ink will cause the problem outlined in issue three above on normal sized text.