Derek is right
This always devolves into the "one versus two" debate.
The rule in the typewriting days was that two spaces were used after so-called "terminal" punctuation, which included:
. period (full stop in Britspeak)
! exclamation point
? question mark
: colon
For commas, semicolons, and single or double quotes you used one space after the punctuation.
I challenge anyone to find original documentation from the 1920s or 1930s that this "rule" came into being because of a general consensus that typewriter fonts, being monospaced, "needed" extra space after a period, space that a proportional font supposedly did
not need. I do not think it had anything to do with the monospaced-versus-proportional-font issue. If anything, it is the
proportional font, not the monospaced one, that would better benefit from having two spaces after a period. After all, proportional fonts are the ones that are scrunched together, not the monospaced ones.
In fact, the "rule" probably came about because some early, influential typing manual dictated the policy. The author of the manual probably thought that the extra white space looked nice. And, in fact, it does. White space always looks good. Try two or three spaces after a period in something written in a
proportional font (the font that supposedly does not need the extra space), and you will be surprised at how nice the text looks.
In casually looking through older texts in proportional fonts, I have noticed that there is often a good bit of white space after periods and question marks and exclamation marks. It is very possible, if not probable, that some typesetting styles mandated extra white space after terminal punctuation, regardless of whether the font was monospaced or proportional.
Spacing of text has not followed a set of consistent rules throughout the history of printing. You, too, have probably noticed that older texts often put a space
before certain marks of punctuation: for example, the semicolon.
As for the original issue of one-versus-two, I have been in both camps, but about twenty years ago I committed firmly to the one-space side. I think it is the best way to go, and I think that typing is now taught that way. I hope to God they are not trying to teach people to type with one space after a period in Courier, but two spaces in Times Roman.
By the way, even in the old days, the two-space rule did not apply to all periods, just to sentences. In abbreviations like *Mr.* you used only one space after the period. However, the "rule" led many careless people to use two spaces after *Mr.* or *Mrs.*, and it looked terrible. Similarly, in certain abbreviations, you had no space at all: Ph.D. and U.S.A.
People who grew up typing the old way (two spaces) are reluctant to change. Fortunately, we can use search-and-replace to change the spacing if we want to.
Roland
My essay on Courier