Everyone (well, everyone I know--my social circles are admittedly very heavy skewed to the language geeks) is loving this article in Slate about how you shouldn't use two spaces after a period. (I will to add to that article an economic argument, which I would bet is the actual reason typographers adopted the one-space rule: Using two spaces after a period means you use more paper, and paper ain't free.)

What really strikes home in this article (and REALLY bothers the author) is how sure people are that there is a two-space rule when there is none:

What galls me about two-spacers isn't just their numbers. It's their certainty that they're right. Over Thanksgiving dinner last year, I asked people what they considered to be the 'correct' number of spaces between sentences.... Everyone—everyone!—said it was proper to use two spaces. Some people admitted to slipping sometimes and using a single space—but when writing something formal, they were always careful to use two. Others explained they mostly used a single space but felt guilty for violating the two-space 'rule.'

This avid belief in rules that do not, in fact, exist is common (and I would argue more harmful) in grammar as well. Grammar education in this country is horrible: When I was attending my (crappy) secondary schools, most years the English teacher would one day say, "OK, we're going to start the unit on grammar!" And the kids would say, "GRAMMAR!?!?! Oh, NO!!!" and the teacher would say, "You don't want to learn grammar? OK, we'll skip that unit." (Imagine how much fun it was to be an English major at Harvard University and then to break into the field of publishing on that kind of sound educational footing!)

Anyway, given this attention to grammar in our public schools, if you ask the average American to name a rule of grammar, they will likely respond with one of two rules, neither of which actually exist.

Ghost Rule #1: Don't split an infinitive. Um, why not? "To boldly go where no man has gone before" sounds a lot better than "To go boldly where no man has gone before" or "Boldly to go where no man has gone before," and it doesn't affect the meaning one bit.

Grammar rules, believe it or not, tend to make logical sense and clarify meaning: You don't say "John and me went to the store" because you wouldn't say "John went to the store, and me went to the store," and you say "I ate only one slice of pizza" because that's all you ate, and "I only ate one slice of pizza" implies that your interrogator is wondering if you, I don't know, had sex with your food first or something. If a grammar "rule" doesn't makes sense or help with meaning, then it's not actually a rule--it may be a preference, but it's more likely a ghost.

Ghost Rule #2: Don't end a sentence with a preposition. Again, there's no real reason not to if it makes your sentence clearer and more concise. It can be more casual, although the fixes aren't necessarily more formal. Which leads me to an inappropriate joke (told to me by my late father, the source of roughly 90% of the inappropriate jokes I know):

A fellow is touring the Princeton campus when he stops an undergraduate. "Hey, where's the library at?" he asks.

The undergraduate pulls himself up stiffly. "Sir," he replies, with great disdain, "here at Princeton University we do not end our sentences with a preposition!"

The man nods. "OK," he says. "Where's the library at, asshole?" 

To take that fix more seriously, the real problem with "Where's the library at?" is conciseness--that extra "at" at the end is unnecessary. But that's not always the case, and pretzeling a sentence around to avoid violating this "rule" can result in horribly awkward prose that was best mocked by Winston Churchill in his famous comment, "This is the sort of English up with which I will not put."

That link will take you to a whole slew of ghost rules for grammar if you want more. What really bothers me about ghost rules is the underlying assumption that grammar is arcane and arbitrary, a collection of mysterious, upper-class, Victorian-era, etiquette-type rules that ordinary people can't possibly understand and won't have much use for. The truth is: Grammar makes sense. It tracks logically, it helps you say what you mean, and because it is so logical, it's easy to understand. It's worth learning because it helps you to communicate to others--and that's well worth doing.