In my career as a writer and editor, I have found that there are two kinds of people in this world: those that put two spaces after a period and those that put only one. I am of the one-space persuasion (which is the standard in the publishing industry).
Why should you put only one space after periods and all other punctuation for that matter? Well, the spacing after a period can affect the “look” of a document. And proper formatting of a document is often as important as the document’s contents—sometimes, even more so.
Typing anachronism
The practice of typing two spaces after a period is an anachronism. It should be tossed out with the typewriter, which was the reason for the two-space policy in the first place.
Typewriters have keys on which the letters are all the same width. The I is the same width as the M, for example. Clearly to anyone with a good eye, an I should be much slimmer than an M. But if the width of each letter was proportional to its ideal width on a typewriter, the keys would be of various widths.
Typewriter manufacturers discovered that if they made all the letters proportional to their ideal widths, the resulting keys would stick together during the typing process. So they made all the keys the same width, and, thus, all the letters were nonproportional to their natural widths. Furthermore, the spaces between the letters all have the same width as the letters. Courier is the standard nonproportional face for typewriters.
Proportional faces
The practice of putting two spaces after a period in this day of proportional typefaces is no longer necessary for a good reason: The designers of proportional faces are true artists. They carefully design into each face the best amount of space for each letter, punctuation mark, and space.
The faces look best with only one space between each word and after each period. Putting additional space after a period not only throws off the balance designed into each face but also leads to rivers of white—vacant space that runs like a river through a paragraph and draws the eye’s attention to it.
Because writers primarily use word processing programs like MS Word that default automatically to a proportional typeface, such as Arial, they should follow the one-space rule. (The one-space rule is endorsed by Chicago, MLA, APA, and Associated Press style guides.)
The complaint that it is difficult to overcome a habit of years does not wash. I double-spaced on a typewriter for years and quickly changed to one space in a day or two once I got my first word processor.
In any case, there is an easy way writers who will not change and their publishers who stipulate one space after a period. The writer can simply type two spaces after every period and then, prior to turning in his or her copy, use the word processor’s find and replace features to automatically substitute one space for each two.