CMYK is a subtractive colour model commonly used in printing. Letters in the acronym stand for the four colours used in the model: Cyan, Magenta, Yellow and… uhm… blacK? The myth goes that using ‘B’ for black would be confusing (on account of blue and brown existing), so powers that be decided to use the last letter of the word ‘black’ in the acronym. This is not quite true.

In fact, K in CMYK stands for Key or Key Plate.

What is a key plate?

The concept goes back to the dawn of colour printing. George Baxter is regarded as the inventor of commercially viable colour printing technique. His method starts from pressing a key plate which defines outline and key details of the image. Once that’s printed, several more blocks proceed — one for each colour — each perfectly aligned (or registered) to the key plate.¹

Similarly in other printing techniques, the key plate was the one with most detail which other plates were registered to. Since the human eye is more sensitive to changes in lightness than hue,² the key plate is usually printed in black or otherwise dark colour.

Do we even need black?

There are a few reasons to include black as a separate ink in the printing process. Probably the most obvious is cost. It’s cheaper to use single ink rather than a mixture of three. Especially as black ink is cheaper than colour ones.³ Cost is hardly the only consideration though.

Inks are physical approximations of idealised primaries. Combining them doesn’t result in a perfect colour mixture.⁴ In practice, layering maximum densities of cyan, magenta and yellow results in a muddy, dark brown rather than a neutral black. It is possible to compensate with calibration, but resulting black would be lighter than one produced by dedicated black ink. That in turn reduces the dynamic range, and thus quality, of the print.⁵

Furthermore, the three colours would need to be precisely aligned. Any imperfection would lead to blurry image especially apparent with text. In fact, with halftone printing method, such flawless alignment is not possible at all. In that technique, the intensity of a colour is simulated by dots of varying sizes printed alongside parallel lines as illustrated in the figure below. To avoid Moiré patterns, for each colour those lines are oriented at different angles. As a result, dots of different colours never perfectly align.

Another reason to use dedicated black is coverage. In print, Total Ink Coverage (or TIC) refers to the cumulative amount of ink applied to a given spot on the substrate (such as paper). For example, a mix of 40% cyan, 50% magenta and 30% yellow results in 120% coverage. Excessive coverage reduces print quality and can damage the substrate. While high-quality paper might handle 300% coverage, cheaper stocks may warp, tear or smudge. Using black ink reduces the TIC improving the reliability of the print.