Some people get their undies all in knot over the silliest things, like how many spaces do you put between sentences.
Traditionally, it’s two spaces after a period, as I was taught in my college typing class. Back then, if you didn’t have two spaces between your sentences in your term papers, some professors would ding you on your grade because your paper wasn’t properly formatted. Even today, most secretaries will tell you two spaces between sentences is the standard, and many teachers still teach students to put two spaces after a period or a colon. It’s been done that way for so long it’s become the norm.
The practice of having two spaces between sentences originated with typewriters, and typewriters placed letters on paper differently than Microsoft Word. Each number and letter key on a typewriter keyboard was connected to a metal type bar, which had the two corresponding letters, one a capital letter, the other a lower case, centered on it. The capital letter was on the top, with the lower case being centered about a quarter inch or so beneath it. The type bars were all the same width, regardless of the width of the individual letters. This caused the kerning, or the spacing between individual letters, to be unequal, which is probably why the practice of putting two spaces between sentences got started.
I’m not going to argue with a secretary about proper etiquette in business letter writing. If that’s what she does for a living then she certainly has more expertise on the matter than I do. However, when it comes to writing manuscripts, the rules change. One space between sentences is all that is required, and two spaces after a period is considered a big no-no.
Typesetting software automatically handles the kerning, or adjusting the spacing between individual letters, so the practice of putting two spaces after periods creates rivers or gaps of white space that weave back and forth across the printed page. This can make the page look visually unappealing, and it’s even more of a nuisance when justifying, or having perfectly even margins on both sides of the page.
Unless you’re an author, the space between sentences probably isn’t anything to be all that concerned about. But if by chance you’re an author, and you’re sending a copy of your manuscript to an agent or publisher for consideration, make sure you have a single space after your periods and colons. Just like my old college professors, they will ding you for not having your work formatted properly, and why give them another reason to reject you.