Royal Blood performs at Slim’s in San Francisco on April 14, 2015. Roman Gokhman/STAFF.

What makes a “guitar band,” anyway? Much has been made of U.K. rockers Royal Blood consisting of nothing more than a rhythm section. There’s drummer Ben Thatcher and bassist-vocalist Mike Kerr. The fullness of the sound the two create by themselves, a thundering herd of cannonball crashes made with effects but no layering, gets equal notice to the lack of their use of the primary instrument of rock: the guitar.

“To me, there’s no difference between a bass and a guitar. I see it for what it is, which is a piece of wood with wires on it,” Kerr said in a quick break just before Royal Blood’s first headlining American tour, which includes April 14 and 15 shows at Slim’s.

A guitar and a bass are the same instrument to Kerr, with the bass having one or two fewer strings and tuned lower than a guitar. Some guitarists play their instrument like a bass, and some bassists play their instrument like a guitar, Kerr said matter-of-factly.

“I’ve got a bass with guitar strings on it, so what do you call that?” he asked. “Two bass strings and two guitar strings; half and half. Is it a bass, or is it a guitar?”

Royal Blood performs at Slim’s in San Francisco on April 14, 2015.

Royal Blood burst seemingly out of nowhere in 2014. The two longtime friends started playing together the previous year. A few days after their first practice, they played their first show. They released several songs, including “Out of the Black,” “Little Monster,” and “Come On Over,” in 2014. Arctic Monkeys’ drummer Matt Helders wore a homemade Royal Blood shirt on stage at one of their concerts. A self-titled debut album was released last summer to critical praise and a Mercury Music Prize nomination.

Thatcher and Kerr earned public approval from Jimmy Page, and Dave Grohl personally invited them to open for the Foo Fighters on their upcoming spring and summer tour. The duo took the invitation as a huge compliment.

“If we could pick any band (for which to open), it would be the Foo Fighters,” Kerr said. “It was a dream come true.”

And when the duo performed in San Francisco last winter, Metallica drummer Lars Ulrich played the part of tour guide, showing them around the city and taking them across the Golden Gate Bridge to Marin.

While success and accolades came quickly, Kerr and Thatcher expected that if they put the effort in, they would eventually reap the benefits. Actually, that’s far from the truth.

“We didn’t think we’d make back the money from our first demo,” Kerr said. “You can imagine our surprise when all these great opportunities presented themselves.”

Royal Blood performs at Slim’s in San Francisco on April 14, 2015.

Both Kerr and Thatcher share a passion for raw, enormous noise influenced by grunge, blues and garage rock, not unlike newer Arctic Monkeys and early Muse. But genre is not something the two spend much time considering as they write. Each song is created in its own vacuum in order to develop individual character and features, Kerr said.

“I would never want to suffocate a good song because I thought it should be a certain way,” he said. “I don’t know what shape it’s going to take.”

But there are a few rules they follow. One of the big ones is that they don’t record anything in the studio that they can’t play live without backing tracks. They use the same instruments, amps, effects pedals and other equipment. Performing as a duo doesn’t mean much if the work can’t be replicated.

Additionally, they have no plans to increase the size of the band. They are happy with the music they make and see no point in expansion. They started a band with friends – each other – and created music with which they are thoroughly pleased.

Mini Mansions perform at Slim’s in San Francisco on April 14, 2015.

Something else that makes Thatcher and Kerr very happy is the new, partly animated and partly live action video for tune “Out Of The Black.” One would suspect that the need for the animated parts exists because acting out portions in live action would not go over well for some younger audiences. Overall, the video features an invasion of aliens disguised in furry costumes who painfully murder customers at a gas station minimart, and a lot of police. It’s the kind of thing that might escape the mind of Muse frontman Matt Bellamy.

In the video created by Adult Swim’s Christy Karacas and director David Wilson, who’s worked with Arcade Fire and Arctic Monkeys, the violence ratchets up quickly to match the frenetic pace of the song itself.

“They presented it to us (and) we thought this was the weirdest, craziest thing we could do right now,” Kerr said. “Weird and crazy is what we like.

“I’m sorry you have to have therapy,” he joked, “but I hope you enjoyed it.”

Follow  Roman Gokhman at Twitter.com/RomiTheWriter.