According to two filings with the U.S. Patent and Trademark Office, the Huffington Post trademarked "H" in late December. One application kind of makes sense: It's for "the letter H inside a square with rounded corners," which does feature a distinctive design element that's recognizable as HuffPo's.
But the other one? "The mark consists of the letter H with a broad solid line below the H." (See the trademark filing at right.) So you can use an H if you like. You could even bold it or italicize it. But just don't underline it.
[Photo of Huffington via Getty Images]