I once read that you could differentiate an urban fantasy from a paranormal romance book by whether the heroine a) slayed or b) fell in love the monsters. Really? I honestly don't think this has ever really been true, because even in the early days of UF, when Laurell K. Hamilton was the only big player on the block, Anita Blake was falling in love with monsters. And most of the PNRs I've read feature strong heroines who do their fair share of slaying.
That's not the only so-called yardstick by which to measure the sub-genres. Problem is, many of the other standard guidelines don't work, either. For example, this one: all paranormals have happily-ever-after endings. What about Yasmine Galenorn? She writes steamy love stories, but doesn't guarantee a HEA. Here's another: PNRs have sex scenes. What about Karen Moning's Fever series? Every bookstore I've been in shelves it in the romance section, but the first few books have zero sex scenes. Zilch! And to contrast that, most of the UFs I read have at least a little smexy-time.
Maybe it's not sex, but love. Urban fantasies only feature flings or crushes that eventually fade. The heroine moves from one man to the next throughout the series, which is in contrast to PNRs that feature one soulmate couple per book—the next book in the series moves on to the next couple. Then how do you explain the Night Huntress books by Jeaniene Frost? Cat and Bones are soulmates who star in multiple books, shelved as PNR. Same with Nalini Singh's Guild Hunter series—several books feature one couple. On the other side of the fence, Stacia Kane's UF Downside series features a heroine, Chess, who is clearly in love with Terrible throughout the series.
The be-all, end-all sub-genre identifier seems to be this: if you strip away the romance from a book, does the story still stand on its own? (If no, it's romance, natch!) I've tried to apply this measurement to my own series, Arcadia Bell. At its core, the series is a story about an occult magician, Arcadia "Cady" Bell, who runs a demon-friendly tiki bar in Northern California. She has a special sight that allows her to identify Earthbound demons—who, to the average person, look like humans. Each of my books features a supernatural mystery that Cady must solve and fight her way out of. But they also feature a strong, soulmate romance between Cady and an Earthbound demon she falls in love with. There are sex scenes, teasing banter, and romantic adventure. If you stripped away Cady and Lon's relationship from the books, the mystery would stand on its own, I suppose, but it wouldn't be half as interesting. And the first two books have HEAs of sorts—hopeful endings in which the couple is together. As far as slaying goes, Cady and Lon both fight the bad guys, usually together. But Lon is demon, so Cady's definitely in love with the monster.
Arcadia Bell has strong, unique world-building and is shelved in urban fantasy, even though a lot of paranormal readers have picked it up for the romance. But in the end, like many other series that cross the line between these two genres, I think people will continue to read it because it doesn't strictly belong in one place or the other. Because sometimes, breaking the rules can be extraordinarily exciting.
Do you have a favorite series that crosses genre lines?