MOUNTAIN VIEW, Calif. — When Dr. Timothy Seelig, the artistic director of the San Francisco Gay Men's Chorus, first heard his organization was being invited to perform with the Mormon Tabernacle Choir, he guessed it was a prank.

"Yeah, when pigs fly," thought the native Texan.

But he soon learned the invite was legitimate — and he was quick to accept.

The veteran choral leader knows music is often called a universal language, but it is also a global unifier. It can bring people together who may seem at odds.

Music's unifying power was on full display Monday here on the stage of the Shoreline Amphitheater in Northern California. Standing shoulder to shoulder for a sound check rehearsal was the Mormon Tabernacle Choir and several members of the San Francisco Gay Men's Chorus.

It wasn't hard to spot the men from the chorus; they were the ones wearing purple T-shirts with the name of the organization emblazoned across their chests.

But otherwise, they blended in with their fellow performers as choir conductors Mack Wilberg and Ryan Murphy led them through segments of songs to be performed at Monday night's concert, the fourth stop of the choir's 2018 Classic Coast Tour.

Chorus member Chris Pettallano expressed a common sentiment heard often Monday: It's all about the music.

"I believe music is universal … so this opportunity is a blessing," he said. "I am not surprised that music can bring two different communities together."

It was 10 years ago that the church that the choir represents supported Proposition 8 in California affirming the definition of marriage as between a man and a woman. The proposition was supported by voters but later overturned by judges. The church was both supported and criticized for its efforts, but it became a wedge issue between many in both the gay community and the church community.

Pettallano grew up Mormon and a musician. He said he remembers listening to the Mormon Tabernacle Choir, "so this is very exciting for me."

Wherever the tour takes the choir and the Orchestra at Temple Square, they look to rehearse with the choral community in the locations where they will perform. So it's logical they would reach out to the San Francisco Gay Men's Chorus.

The 2018 Choir Tour includes three Bay Area venues. The San Francisco chorus — which features some 300 voices — has been touring and recording for some four decades.

"The purpose of music is to unite people," said Mormon Tabernacle Choir President Ron Jarrett. "Music affects people in many ways. This is a great opportunity to bring two cultures, if you will, to the center of things and to build bridges, to make friends, and to make an enjoyable experience for everyone through music."

Since its beginning, the Mormon Tabernacle Choir has functioned as an ambassador for The Church of Jesus Christ of Latter-day Saints. That charge was evident Monday. "The chorus members are having a glorious experience, as are our people," Jarrett said.

Music has been a part of Jarrett's life since he was a boy. Monday was a reminder to the choir president of music's power.

Seelig has been invited to take the baton at some point during Monday night's concert and lead the choir and orchestra as a guest conductor. With years of conducting experience, he will be very comfortable in front of the iconic choir. Still, he counts Monday as an unforgettable moment in his musical career.

"I had a blast; who gets to do that?" he said after conducting the choir during the afternoon rehearsal.

Seelig called the afternoon "a milestone for these two organizations that we did not think would be coming for maybe 10 or 20 years."

He called the Mormon Tabernacle Choir "the greatest choir in the world, hands down."

Meanwhile, the chorus he directs is the world's oldest gay men's chorus. "We are the granddaddy of the LGBT choral moment, what we do is build bridges."

Seelig acknowledged "obvious differences" between the two organizations, "but when we get to sing together it doesn't matter. We are blending our voices to make music as beautiful as we can."

He said he felt some pushback when people learned of the choir's invitation. "It doesn't heal everything, but it is a step on the part of the Mormon Tabernacle Choir to open the doors for this rehearsal to the Gay Men's Chorus."

Monday's sound check rehearsal was serious business as performers and directors worked together to find the perfect sound for Monday night's audience. But there were still moments of laughter as two groups brought together by music joined their voices as colleagues.

Chorus member Zach Herries was initially hesitant to perform alongside the Mormon Tabernacle Choir. But he was happy to take a seat Monday rehearsing near the front row with the choir.

"I grew up Mormon, so this is part of who I am and I am grateful for this opportunity."