Why all the hate? --Two spaces after the period

Question

The nuns enforced it with a ruler: two spaces after a period at the end of a sentence. I, along with many others do this instinctively. Why is this considered incorrect/improper in printed materials and on the web?

Answer 1

Two spaces after a period at the end of a sentence started when the typewriter replaced hand set printing presses. When type was set by hand the spacing was carefully crafted to make sentences and paragraphs easier to read. Typewriters use a monospace font that make it hard to distinguish the end of a sentence without adding the extra space.

Modern fonts are kerned so they don't need the extra space at the end of a sentence. In HTML adding the extra space requires using encoded HTML entities. You can hit the spacebar as many times as you want but you only get one space.

If your using a monospaced font add the extra space otherwise it's just not necessary.

---

Monospaced examples:

Double-spaced

Lorem ipsum dolor sit amet, consectetur adipiscing elit.  Proin sagittis urna
ut tellus fringilla malesuada.  Fusce rhoncus varius nisi ut interdum.  Vivamus
rutrum tempor risus aliquet rutrum.  Nunc ornare luctus cursus.  Sed lacinia
ultricies mi, eget vestibulum mauris tempus et.  Suspendisse pretium pharetra
ornare.  In porta rutrum orci at malesuada.  Maecenas nisl arcu, commodo nec
aliquet id, pulvinar non dui.  Sed ac ultricies odio.  Etiam tellus nunc,
lobortis id luctus vel, pulvinar non risus.  In sollicitudin eros nec sem
suscipit feugiat accumsan pellentesque nisl.

Single-spaced

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin sagittis urna
ut tellus fringilla malesuada. Fusce rhoncus varius nisi ut interdum. Vivamus
rutrum tempor risus aliquet rutrum. Nunc ornare luctus cursus. Sed lacinia
ultricies mi, eget vestibulum mauris tempus et. Suspendisse pretium pharetra
ornare. In porta rutrum orci at malesuada. Maecenas nisl arcu, commodo nec
aliquet id, pulvinar non dui. Sed ac ultricies odio. Etiam tellus nunc, lobortis
id luctus vel, pulvinar non risus. In sollicitudin eros nec sem suscipit feugiat
accumsan pellentesque nisl.

Answer 2

From a newsletter I sent out to author clients a few years ago (edited to remove monospace information already covered in Chris's answer):

When people made the switch from typewriters to word processors and proportionally spaced type, the term "typing" stayed with us. It makes sense: "I'm word processing a letter" is a clumsy mouthful, so the simpler, more traditional term stuck. The problem is, when you use almost any font on a computer you aren't really typing, you are really typesetting, a very different thing.

The art of typesetting proportional fonts is handled automatically by the computer (often poorly, but that's a story for another time) as you type. The point is, what you are doing is typesetting, not typing, and nobody bothered to let you know.

That's a shame, because the result is that many things typesetters (a highly paid, highly skilled profession) take for granted have never been communicated to the present generations of computer users. Worse, the typing practices that are still taught in schools are just plain wrong, unless you are actually using a typewriter (or a font that mimics one, such as Courier).

Spaces Don't Come In Twos

Many, many computer users automatically put two spaces after every period, colon, or closing quotation mark, not realizing that the space designed into punctuation already takes into account the extra gap needed after the end of a sentence, so one space, not two, is correct. This has been standard typesetting practice for several hundred years. The Chicago Manual of Style says: "In typeset matter, one space, not two (in other words, a regular word space), follows any mark of punctuation that ends a sentence…" without mentioning the reason. Typeset matter, as a glance at any magazine or book will tell you, is always proportional-spaced. Like I said: you're typesetting, but nobody bothered to tell you.

In typewritten text, two spaces help to make the text more readable by providing a visual clue to the beginning of a new sentence. In typeset text, two spaces not only don't improve readability, they also create random blobs of white space that make the text look subtly but definitely wrong, (especially when you know the difference). Here is some text set with two spaces between sentences, then with one. The typeface is Tunga Regular.

[Added later] All that said, it dawned on me after I wrote the above that there is a much more fundamental reason why one doesn't use two spaces after punctuation, and is the reason for the single-space rule. Since before there was such a thing as movable type, scribes and typographers have strived to achieve even typographic color on the page. If you're not into type, you probably don't think of plain black type as having "color," but the term refers to the relative grayness or blackness of a page or a paragraph. Even color (pick up any good book, open to a text-intensive page, and defocus your eyes a bit) is one sign of good typography. Uneven color is distracting and detracts from the readability of text, and makes the page less inviting to read (another goal of typography).

The "one space" rule is there because it maintains that evenness of color.