Remember the days when we learned to type in school? “Remember, kids, the way you break up a sentence when you type is by putting two spaces after a period. Got it?”
Those days are over. Today, every major style guide, including The Modern Language Association Style Manual, the Oxford Style Manual and the Chicago Manual of Style, calls for one space between sentences. Period.
The “two spaces” thing dates back to manual typewriters. Typewriters have inconsistent spacing, and they require two spaces in a break in order to see the break at all.
Nobody uses manual typewriters anymore.
Now that we’re all using computers that are capable of proportional spacing, we don’t need to use two spaces. In fact, it’s better if we don’t. It looks cleaner, it may enhance readability, and it’s less work. Sure, we’re just talking about one extra push of the spacebar, but when we’re all in danger of suffering from carpal tunnel, every little bit helps.