Fans of Kacey Musgraves have undoubtedly noticed a special hidden duet with Willie Nelson at the end of her recently released album, Pageant Material. The track is a cover of Nelson's decades-old tune "Are You Sure," and Musgraves says that the idea to play it together came up when she was opening for the iconic singer on tour last year.

The two were on Nelson's tour bus when Musgraves asked him why he never plays the song live these days -- and the question surprised Nelson.

"He couldn’t believe I knew it,” Musgraves tells Radio.com. “I just really loved it. It’s very honest and very country, [and] it’s a neat perspective. He seemingly pulled a guitar out of a cloud of smoke and started strumming. I was of course dying inside a little bit.”

One thing led to another, and the two singers ended up recording the song together, with Nelson using his iconic guitar Trigger to play the tune.

“It was meant to be a hidden track at the end," Musgraves says. "A little nugget for people who make it to the end of the record.

Nelson has been one of Musgraves' biggest supporters in the industry, and, prior to hitting the studio with the "Biscuits" singer, the country legend said that he was looking forward to recording the track with her.

“This is the one she wanted to do,” he said at the time. “It’s one of those obscure songs of mine that I wrote many years ago in Nashville ... I’d love to record with her. She’s a great artist.”

Pageant Material is available on Amazon and iTunes.

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