In just five years, Pentatonix has sold more than six million albums in the United States alone and performed for hundreds of thousands of fans at sold-out shows around the world.

The five-some, Scott Hoying, Mitch Grassi, Kirstin Moldanodo, Avi Kaplan and Kevin “K.O.” Olusola, got its start after appearing on the reality TV competition show, “The Sing-Off,” devoted solely to a cappella groups. Pentatonix won the competition and was awarded $200,000 and a recording contract.

“Sometimes, people are like, ‘This isn’t my thing.’ But I would like them to recognize that we are trying to accomplish beautiful and impressive harmonies that bring true musicality to our music. We work really hard to be something different and new. It’s only us and our voices,” Hoying says in a phone interview.

Pentatonix, the three-time Grammy Award-winning a cappella group, performs at 7 p.m. Sept. 2 at The Great Allentown Fair.

The group’s members met in Arlington, Tex., where Hoying went to high school with Kirstin Maldonado and Mitch Grassi. Through a mutual friend, they met Avi Kaplan, bass singer, and then linked up with Kevin, “K.O.” Olusola when the Yale University grad’s beat-box videos went viral. Olusola chose Pentatonix over medical school, a choice his Nigerian immigrant parents were not originally on board with.

“We do everything as a group. All of us write and all of us arrange. We all pitch ideas and bring our thoughts to the table. We all have different musical talents and it’s cool that we can find our common ground and bring it together. That common ground is Pentatonix” says Scott Hoying.

Pentatonix, a name suggested by Hoying, is named after the pentatonic scale, a musical scale or mode with five notes per octave, representing the five members of the group. The last letter was replaced with an ‘x’ to make it more interesting.

After Pentatonix was dropped from Epic Records in 2011, they found a new audience with their YouTube covers of Top 40 hits, singing songs like Lady Gaga’s “Edge of Glory” and Nicki Minaj’s “Starships.”

Pentatonix made a five-minute, seven-song futuristic video of Daft Punk medleys that led them to win the 2015 Grammy for Best Arrangement, Instrumental or A Cappella.

The group’s 2015 self-titled album was certified gold after debuting at No. 1 on the Billboard Top 200. The Pentatonix YouTube channel boasts close to 13 million subscribers, more than Lady Gaga, Daft Punk and even Beyoncé. It has yielded more than two billion video views.

Pentatonix starred in its first NBC network Christmas special last December, “A Pentatonix Christmas Special.” During the two telecasts, the show garnered 13 million views. The special featured music from all three Pentatonix holiday albums.

“A Pentatonix Christmas,” which was certified platinum and spent two consecutive weeks on the Billboard Top 200 Chart as well as reaching No. 1 on iTunes and Amazon, includes a cover of the Leonard Cohen classic, “Hallelujah.” Pentatonix’s cover of the song reached No. 2 on the iTunes singles chart and generated 20 million video views in its first week, more than 130 million views total. It has since been certified gold.

The album followed the 2014 Double-Platinum album, “That’s Christmas To Me,” which features the group’s Grammy Award-winning arrangement of “Dance of the Sugar Plum Fairy.” In 2015, the group released its first tour documentary, “On My Way Home,” and appeared in the feature film, “Pitch Perfect 2.”

“Christmas is something that we love a lot and, from the beginning, the group wanted to do a Christmas album. Christmas music goes very well with a cappella and there is a nostalgia to it. Sometimes, people ask us what we are going to do when we run out of Christmas songs to sing, but there are so many Christmas songs out there and people love it, so we will keep singing them,” Hoying says.

The group’s newest release, “PTX Vol. IV - The Classics,” features its Grammy Award-winning performance of “Jolene” with Dolly Parton, as well as an arrangement of the John Lennon classic, “Imagine.” The group is working on Christmas music right now and is starting the process for its next album.

“Last year, we were on a 20-country tour, so this year we stayed in America and gave our poor voices and bodies a break. It was unbelievable and amazing, but we were worn out, as amazing as it was,” says Hoying.

“The show will be a lot of fun. It’s a combination of our original and cover songs from YouTube. We implement a lot of songs from the new LP, ‘Classics,” says Hoying of the Allentown Fair Grandstand show.

Opening the concert is Us The Duo,

Tickets: allentownfairgrounds.ticketoffices.com; ticketmaster.com; Allentown Fair Box Office: 610-433-7541.