Samuel Adams recently promoted its Double Agent IPL to year-round availability. (Boston Beer)

There’s something disconcerting about India pale lagers, or IPLs. They’re the rebels, the heretics of the beer world. They don’t play by the rules.

Ever since the late 19th century, when it became possible to culture pure yeast strains, malt beverages have been divided into two kingdoms: lager and ale. Lagers undergo a leisurely, cold fermentation with yeasts that produce a mellow, even-keeled, clean-tasting beer. They’re hopped with varieties noted for their delicate, dry spiciness: so-called “noble” hops.

Ales undergo a vigorous, short, warm fermentation that produces a fruitier, spicier, funkier brew. They can tolerate a more aggressive hopping with strains full of earthy, resiny, floral and citrusy notes.

IPLs enjoy dual citizenship. They’re fermented like a lager, then hopped with pungent Pacific Northwest hops well past the point that purists would consider proper for a lager.

These hybrid brews have been around awhile. Jeremy Cowan of New York’s Shmaltz Brewing introduced Sword Swallower six years ago as part of his line of freak-show-themed Coney Island beers. “I called it an India pale lager, but nobody accepted the category back then,” he says. Not everybody accepts IPL as a style today. lumps such beers under the heading “strong pale lager/imperial pils.”

Nevertheless, these category straddlers are proliferating. “Everybody had the same idea at the same time,” says Favio Garcia, co-wner of Lost Rhino Brewing in Ashburn. Locally, the District ChopHouse in the District; Devils Backbone in Roseland, Va.; Wild Wolf in Nellysford, Va.; and Rock Bottom Brewery in Bethesda have experimented in making lagers with ale hops. Nationally, Widmer Brothers Brewing in Portland, Ore., released its Hopside Down as part of its “Rotator” series of IPAs, and Boston Beer has promoted its Samuel Adams Double Agent IPL to year-round status. International examples include Mikkeller Hop Burn Low from Denmark and Nogne O India Pale Lager from Norway.

Lost Rhino’s entry is Megafauna (translation: “large animal”). Twenty-five cents from the sale of every pint, growler and 22-ounce bottle sold in the brewery tasting room will be donated to Save the Rhino International, promises Garcia. The kettle hops for this strong lager (7.5 percent alcohol by volume) are the three “big C” strains — Columbus, Cascade and Centennial — commonly found in American ales, but brewer John Peters dry-hopped the beer with German Hallertau and Saaz. This result is a brew that starts out with a grapefruity tang, then segues into the long, dry, peppery finish that you’d expect to find in a first-rate Pilsener. Megafauna is available on tap in the District and Virginia and in 22-ounce bottles only at the brewery.

The partnership of lager yeast and ale hops isn’t an equal one. In Great Lakes Brewing’s Silver & Gold IPL, “the IPA definitely overshadows the lager part,” says Lauren Boveington, the brewer’s District-Northern Virginia sales rep. The Cleveland brewery released the hybrid brew in June to celebrate its 25th anniversary. Brewer Luke Purcell fermented it with the same yeast he uses for Great Lakes’ Dortmunder Gold; the hop bill includes Calypso, a strain that contributes notes of apple, pear and tea.

IPLs shouldn’t numb your palate with hops and alcohol the way many imperial IPAs do. Ideally, the clean lager finish should serve as a canvas to show off the subtleties of the hops. Boston Beer adds more than a half-dozen varieties to its Double Agent IPL. The centerpiece, says company chairman Jim Koch, is a relatively new strain called Mosaic, equal parts floral and fruity. Double Agent also contains American Citra (a variety with a distinctive tangerine flavor) and Nelson Sauvin (a New Zealand hop often compared to litchi and sauvignon blanc grapes). They combine to give the beer a soft, fruit-cocktail type of finish.

If you can brew a lager with ale hops, why not an ale with lager hops? That niche, too, has been filled: Stone 17th Anniversary Gotterdammerung IPA is a West Coast IPA hopped to the hilt with varieties from Germany, a land that traditionally has prized balance and drinkability over all. Gotterdammerung has a clean, intensely bitter, long-lasting aftertaste, with notes of black pepper, anise and a little citrus.

Reaching such a level of hoppiness with traditional European strains would have been nearly impossible. But the Germans, perhaps recognizing that change is in the air, have been breeding new varieties with a much higher percentage of alpha acids, the primary bittering compounds in hops. Gotterdammerung incorporates several of those newcomers, including Herkules, Merkur, Opal and Smaragd. Clocking in at nearly 10 percent alcohol by volume and 102 bitterness units, this is the sort of beer that brings an evening to a close; a name that translates as “twilight of the gods” seems somehow appropriate.

Will India pale lagers (or German IPAs, for that matter) ever become a recognized style? That remains to be seen, but they do raise a philosophical question. If brewers can trespass across stylistic borders with such impunity, do we need a whole new taxonomy for beer? Is the very notion of beer style outdated?

It’s about 30 years too late to summon the style police, suggests Koch. “Ever since the early 1980s, U.S. breweries have basically been reviving old World styles and putting their own twist on them.”

Take IPAs. “They’re no longer sent to India, and they’re not always pale. When you start getting all these black, red and white IPAs, you have to ask yourself, ‘What does an IPA mean?’ The common element is a big dose of high-alpha-acid American hops. So why does it have to mean an ale? Let’s try it with a lager!”

As Lost Rhino’s John Peters notes, “American craft brewers bend the rules and throw the book out the window.”

Kitsock is the editor of Mid-Atlantic Brewing News.