Why is black in CMYK called key?

Answer 1

CMY-KeyThe black plate is called the key plate because in modern four-color printing (also called process printing) the cyan, magenta and yellow colors are keyed, or aligned, with the black key plate.

When printing is done using metal plates, the black plate is called the Key plate. This is because it contains the artistic detail or "key" information.

It eliminates confusionA fun but unlikely explanation:

In the really old days of printing, apprentices were called printer's devils, and one of the devil's jobs was to load ink into the press--a very nasty, fairly dangerous task. When process printing came along, very few people had multi-unit presses--you'd put down yellow first, then cyan, followed by magenta and finally black. (If you DID have a multi-unit press, it could only print two colors at once and process work requires four colors.) Every time you finished one color, you had to take the plate out of the press, clean it, coat it in gum arabic, then scoop the ink out of the fountain, wash the press, and reload everything for the next run. It only sounds like a time-consuming pain in the neck because it is. Plus you'd have to register the press to the colors that were already printed and get the color set right. If you're running four colors on both sides, you're looking at several days of work.

So check it out. You are a devil working with a printer who's running 25,000 impressions. You got to work at 6am and have been toiling all day. At 8pm, your boss tells you, "Devil, ink up the press for the last color and we'll get started." Now here's the situation: You've been up since 4:30. You had to walk five miles from the printer's basement to your post. You've been sniffing solvent all day so you're high as a kite, you've been doing very heavy work and the last time you saw food was at noon. So you scrub down the press, hang the plate, look at the color label: "B." You stagger, whether from lack of food or lack of oxygen, to the ink room and grab the first can that has a "B" on it. By now you can ink the press with your eyes closed which is good because you can hardly see the press even though it's painted white, and the end the ink goes in isn't lit that well in the first place, so you open up the can, grab your ink knife...and trowel five pounds of blue ink into the press.

This happened. A lot. And it was legal to beat devils.

To keep it from happening any more, the printer's union decided black ink would be abbreviated "K." They later decided "K" stood for "key" because everyone kept asking, "why are we using 'K' for black?"