When the first Macintosh went on sale in 1984, its screen was built to a very particular number: seventy-two pixels to the inch. That figure was not an accident of the hardware. It was a deliberate handshake with the printing trade, which had measured type in points for centuries, at seventy-two points to the inch. By matching the two, the Mac struck a bargain so clean it barely needed explaining. One point on the page would be one pixel on the screen.

The payoff was WYSIWYG, what you see is what you get. Set a headline in twelve-point type and it stood twelve pixels tall on the display, so the document on screen was a faithful preview of the document that would come out of the printer. The screen stopped being a rough approximation of the page and became a window onto it. For anyone who had wrestled with earlier machines, where the relationship between the glowing text and the printed result was a matter of guesswork, this was a small revolution in trust.

That old bargain never really went away; it just got more complicated. Every argument since about logical pixels, about display scaling, about the doubled-up dots of a high-resolution screen, traces back to the moment someone decided a pixel and a point should be the same size. Modern layout still negotiates with that decision, even as physical pixels have shrunk far below anything the eye can pick out. The number seventy-two is mostly a memory now, but the promise it made, that the screen should tell the truth about the page, is one the web is still trying to keep.