Chances are, sometime in the past month, you’ve probably sung a Christmas carol. Or at least caught yourself humming one.

But you probably didn’t think much about where that carol came from.

“Carols are far more diverse than people think,” says Paul Laird, a professor of music and dance at Kansas University. “A lot of the famous ones are the newer ones, from the American popular music machine of the 20th century. But now we sing those right next to the much older ones.”

While many people auto-pilot their way through most of the holiday repertoire, they might not realize that many of those carols have interesting stories behind them.

Take, for instance, “Silent Night.” It was intentionally written with simple chord progressions to be played by a guitar. Legend has it that the organ didn’t work at the church where German composers Franz Gruber and Joseph Mohr were to perform in 1818, so they needed a song to be accompanied on guitar.

Or “O Little Town of Bethlehem,” written in 1867 by American Phillips Brooks. He was inspired to write the piece by riding from Jerusalem to Bethlehem on horseback.

Some songs we now know as Christmas carols are really religious poems set to secular tunes. One of those is “Hark, the Herald Angels Sing,” with words by Charles Wesley. The tune was originally written by Felix Mendelssohn, as part of an 1840 cantata honoring Johannes Gutenberg’s invention of the printing press.

Another example of that is “What Child is This.” It’s set to the tune “Greensleeves,” a love song written during the Renaissance.

Meanwhile, “Joy to the World” shows an old version of what’s known as “sampling” in today’s hip-hop music world. American Lowell Mason took the first four notes of the “Lift Up Ye Heads” movement from Handel’s “Messiah” and turned them into the first four notes of “Joy to the World.” Most hymnals still attribute the carol to Handel.

John Buehler, chairman of the music department at Baker University, notes that carols are a broader category of music than just Christmas tunes. They’re brief pieces sung with multiple verses, often originally meant to accompany dancers.

Many didn’t survive. He thinks one common denominator for the ones that are still around is how easily they are sung.

“Undoubtedly, one of the criteria of quality, if standing the test of time is part of that, is compelling wedding of text and tune,” Buehler says.

But Craig Parker, a music professor at Kansas State University, says it might be more complex than that.

“You would think singability would be one thing,” he says. “But ‘Silent Night’ has a pretty huge range – it’s only one note less than ‘The Star-Spangled Banner.’ They say ‘The Star-Spangled Banner’ is unsingable, but they don’t have a problem with ‘Silent Night.'”

Parker says he thinks today’s standard repertoire of Christmas music is guided by nostalgia more than anything else. Adults want to hear the carols they heard as children.

“They evoke an image in your mind, of being with family and thinking about long ago when life was easier,” Parker says. “I’ve done church music on and off in my career. There are certain (carols) people like, and if you don’t sing them, it’s, ‘Why didn’t we sing so-and-so and why did we sing this one nobody knows?'”

For that reason, he says, it’s easier for a secular Christmas song to make its way into the seasonal cycle than it is for a sacred tune.

So this Christmas, think about those time-tested carols and where they came from. Or don’t – they still can be meaningful.

“I think, quite honestly, even I don’t think of that much,” Buehler says. “Christmas carols fit with a season and not an era.”