Mark Gatter, in the book he published titled “Getting it right in print”, states the “key plate” is used align the other plates, but does not mention why it is called “key”. While the term today may have adopted the meaning for the process of aligning plates, the term “key” does not originate from this process. The color “registration” is more closely related to the process of aligning plates. The term “key” originates from the rotary printing press in 1843 that had screw keys to control the amount of ink printing on a substrate.
Offset presses also use this mechanism. Most other countries who speak languages other than English typically used “CMYK” to mean four color process; even if the individual colors don’t begin with the same letters. However, in French speaking countries, the accronym is “CMJN” is used meaning “Cyan, Magenta, Jaune, Noir”. “Jaune” and “Noir” translate to “Yellow” and “Black” respectively. This shows that in certain countries, the model directly translates to “black” and not “key” or any other term referring to alignment, detail, or contrast.
While Johannes Gutenburg invented the printing press in Germany around 1454 which revolutionized moveable type, it was Jacob Christoph Le Blon who invented three-color and four-color process printing around 1725. What is interesting is that his original color models were RYB and RYBK. The argument here is that the color blue (represented by “B”) is used in the same process as black, so the initial may have been changed to avoid confusion.
His three-color process would have used the same aligning technique as his four-color process, so if “key” was meant to mean “key plate”, it would theoretically have been the blue plate in his three-color model. While it may never be known the exact origin of the use of “key”, it can be discerned that the myth could perhaps be true and that the origin of the term could have been to simply avoid confusion with the color blue. It appears the context clues may outweigh the notion that K is used to denote “detail” or “contrast”.