For those of us old enough to remember typewriters and the convention of adding a double space between sentences, the question of one space or two often arises. The answer now is, unequivocally, one space, but why the change?
The convention of adding two spaces between sentences arose because this rendered text easier to read when typed out: with only one space, sentences would appear to run into each other. However, when typeset into a book where paragraphs are usually justified, this convention becomes irrelevant: words, and sometimes individual characters, are spaced out (“kerned”) to make them fit on the line, creating variable widths between words and also between the end of a sentence and the beginning of the next. (If you open any book and look closely at the text you will see how words and sometimes characters are spaced out differently from line to line.)
With the advent of digital word processing (and desktop publishing), text is auto-kerned by the program. Word processors treat all spaces as characters; thus, if you input a double space after the end of a sentence in a paragraph with justified text, this can create odd or exaggerated kerning, often resulting in unsightly large spaces between sentences.