Why You Should Use One Space After Each Sentence

This document asserts a very simple rule: press the spacebar only once at the end of each sentence.

This isn't too hard of a concept to grasp. Although, the question is: why?  Why not use two spaces?

After years of using a typewriter, and learning to put two spaces after every sentence, why should I change?

There are two reasons.

The first reason is because typewriters traditionally use non-proportional fonts; every character is the same width. Consider a word like illumination. A lower-case l or i takes up the same space as an m or even M. 

Because they use non-proportional fonts, our minds can separate the sentences easily only if there is quite a lot of space between the sentences.

But when you use a word processing program, like Microsoft Word, you generally use proportionally spaced fonts. So in a word like "illumination", i's and l's are much skinnier than an m. Because the letters aren't spaced out so much, our minds can cope with less space between sentences. And text in proportional fonts, with an extra space at the end of the sentence, looks too chopped up.

The other reason is that some people like to justify text. That means that Microsoft Word will put little spaces between the words to stretch out the text so that the right-hand margin is also straight. Publishers typeset most books justified. Microsoft Word uses programming to to stretch each line so that the right-hand margin is also even. If you have extra spaces in the middle of sentences, Microsoft Word stretches the text in each line and includes your "extra" spaces as well.

So: when using Microsoft Word, use one space at the end of every sentence.

Ref: http://www.shaunakelly.com/word/concepts/rules_onespace/index.html