Put one space after each sentence when using proportionally-spaced fonts, which means just about every font found on a computer including the one on this webpage (notice that an i takes up less than half the character-space width of an m). Put two spaces after sentences when using mono-spaced fonts, such as Courier New and those found on a typewriter. With mono-spaced fonts, an i takes up the exact same character-space width as an m. Reading documents written with mono-spaced fonts becomes, well, monotonous, so someone long ago decided to put two spaces at the end of each sentence to make it more readable and to act as an “eye rest.” It’s hard for some people to make this change because they’ve been putting two spaces at the end of a sentence forever. But if you need final proof of this rule, open any book, newspaper or magazine: you’ll find only one space at the end of each sentence.