"A Negative x A Negative = A Positive"

What has math got to do with grammar? Well, in this case, lots. The commandment of "Thou Shalt Not Use A Double Negative" is a well known rule of grammar, and, in the big scheme of things, it's a fairly easy one to remember and practice.

Simply don't use more than one negative expression to convey a negative idea in a sentence. Words such as only, scarcely, never, and but have negative meanings. Therefore, don't combine them with any other negative word, such as not, if you want to retain the negative meaning.

Woe is I tells us a little about the history of the double negative rule. O'Conner points out that "For centuries, it was fine to pile one negative on top of another in the same sentence. Chaucer and Shakespeare did this all the time to accentuate the negative. It wasn't until the eighteenth century that the double negative was declared a sin against the King's English, on the ground that one negative canceled the other" (188). She also says that the guy responsible for this rule was Robert Lowth, the same character who decided in his infinite wisdom that we shouldn't end sentences with prepositions!

When I taught language arts, one of my favorite examples of this was the commercial slogan "Nobody Doesn't Like Sarah Lee." I pointed out that if "nobody doesn't" like her products, then, of course, that means that EVERYONE DOES like them. When we finished language arts and worked on math, I reinforced the concept by pointing out that a negative x a negative did, indeed, = a positive. Isn't it nice when you can drive children crazy in every subject?

I don't think double negatives are always bad. Sometimes it's important to be able to avoid being blatantly negative. Woe is I gives two examples that everyone (especially every man) should memorize: "Your blind date is not unattractive" and "I wouldn't say I don't like your new haircut" (189). As you can see, tact occasionally trumps grammar in the real world.

For some fun exercises and examples of the rule, visit Skillwise and LEO. Teachers will find The Teacher is Wrong site full of fun and interesting ways to teach this concept.