CMYK

What the word means

A screen makes colour by adding light. It starts black and shines red, green and blue at you, and all three at once give white.

Paper does the opposite. It starts white, and every ink laid on it takes light away. Three of them — a blue-green, a pink-red and a yellow — cover the range between them, and together they ought to give black. In practice they give a muddy brown, so a fourth ink, black, is printed as well. It also saves the other three on every letter of text, which is most of what a page carries.

The fourth is the one the last letter of the name stands for. It is called the key plate: it carries the detail of the page, and the other three are lined up against it.

What it does on a real document

A colour in a document meant for printing is written as four numbers, one per ink, rather than as three for a screen. The strictest printing rules accept nothing else.

A document written in screen colours and sent to a press is converted by somebody, somewhere, with settings you never see. Stating the inks yourself is what stops the surprise.

Where it comes from, and why

Printing in three colours rather than one is centuries older than any computer. It follows from an old discovery: most colours can be mixed from a small number of others.

The black plate came from the workshop rather than from the theory. The three together were never quite black, black ink is cheaper than three, and text printed with three plates goes fuzzy the moment they are a hair out of line. The four letters became a habit of the trade long before the PDF format, which simply carries what the trade already spoke.

See it on a finished document

Grey, screen and printing colours A sheet as it goes to the press The rules behind this word, and the texts they are read from