Print media evolved into its present forms.
In, say, 1469, there were no page numbers. This obvious and now necessary part of the book's user interface simply did not exist.
The earliest extant example of sequential numbering in a book (this time of 'leaves' rather than pages, per se) is the document you see at the top of this page, Sermo in festo praesentationis beatissimae Mariae virginis, which was printed in Cologne in 1470. The practice didn't become standard, the wonderful I Love Typography tells us, for another half century.
The page number is particularly interesting, I think, because it is a pointer, a kind of metadata that breaks apart a work into constituent parts. The existence of page numbers creates a set of miniature sub-publications to which someone can refer.
Now, books can be sliced and diced in an ever-expanding number of ways by computers. In a recent book I read in the Kindle app on my phone, a highlight I made came at location 7525. There were 8958 possible locations. These are really the page numbers of the e-book age. We're still living with the coarse resolution that the print formats provided, but that's changing.
Google Books is carrying on the tradition of this 15th century German printer, too. The precision of its pointers has extended down to any individual word or sentence, as well as strings of phrases held together by search engine code.
Web pages, too, are slowly getting more precise pointers, this extended metadata. Dave Winer has been allowing people to point to individual sentences on his blog forever, and the Times has experimented with it in the past. Obvious Corp's Medium and Atlantic Media's Quartz allow for paragraph-level commenting.
But check out the HTML on the New York Times these days:
<p class="story-body-text story-content" data-para-count="178" data-total-count="1201" itemprop="articleBody">Mr. Zuckerberg, by contrast, has built his immensely profitable empire by collecting reams of personal data on his 1.2 billion customers and using that to sell ads aimed at them.</p>
<p class="story-body-text story-content" data-para-count="265" data-total-count="1466" itemprop="articleBody">Even as Facebook grows more corporate and more complicated to use, Mr. Koum and Mr. Acton, 42, are obsessively focused on just one thing: offering a simple, private, nearly free way for people to share text, photo and video messages with the people they care about.</p>
With those numbers after the paragraph markers, The Times can track how many characters a visitor has read. The Times stopped paginating stories, one of their designers told me, and this system is the elegant replacement. It lets them show a reader an ad every X characters.
The advertising inventory available in a publication used to be denominated in pages. Now, it's in characters counted by software.
The page, as a unit of text, has proven durable, but its reign is ending as its purposes are unbundled and farmed out to software.
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