To understand the role that music played in the 20th century social transformation of our nation, you just can’t ignore folk music.

But beware that, when you examine it, you’ll be hard pressed to define it, let alone place it neatly in the scheme of things.

Start with a definition. “Folk” music is generally defined as the music “of the people,” but that in itself isn’t necessarily a compliment. Any number of music historians have termed it the music of the uncultured.

It is also generally described as music originating in popular culture and passed generation to generation, and that’s important quality. And it usually has its roots in stories of unknown origin. Modern “folk” lyricists sit down and write song-stories, but in its historic form, the music usually came down via tradition with no known author.

In the U.S., much of our music tradition — our folk tradition —naturally comes from immigrants. Scottish and Irish immigrants who settled in the Appalachian Mountain range brought with them the instruments, style and beloved musical stories of their homeland.

Alongside them, African Americans brought a rich tradition of music from heir homeland as well as Afro-Caribbean music they developed along the way. That evolved into African American gospel and blues that had an incredible impact on our music tradition.

Folk music always was, and remains, an outpouring of happiness, pain and a yearning to build a better life. And the latter, in particular, describes its greatest 20th century role in America. For “folk” music became the messenger for change, a voice often crying in the wilderness for racial justice, for worker rights, as well as environmental protection and a sensitivity to the land we inherited.

Folk musicians have often been seen as radical, and sometimes they were.

Standing head and shoulders above many of them was Woody Guthrie. He was sympathetic with the ostensibly high principles of communism, though he was never a member of a communist organization. That sympathy tainted his public career, but he never back away from it. When he performed publicly in the 1940s, he played a guitar with a small sign on its face reading “This machine kills facists.”

Guthrie’s most enduring song is still a standard wherever folk music is appreciated, and millions of Americans can sing the lyrics to “This Land Is Your Land.”

Guthrie’s health was sharply declining by the early 1950s when a rebirth of the folk tradition took hold and began supporting social change.

A quartet known as The Weavers was forceful in its call social justice until Sen. Joe McCarthy’s Red Scare took hold and the group, threatened with being blackballed, eased its message to remain in business. Its most enduring song is “Good Night, Irene.”

One of the Weaver’s members in the early days was Pete Seeger, and he picked up the mantle dropped by Guthrie and wore it until his death. His “Where Have All the Flowers Gone?” became a theme ballard of the anti-war movement of the 1960s, but he is probably best remembered for his environmental fervor. He founded the Clearwater movement on the Hudson River and spent his later years pushing for environmental protection.

Nor can the Carter Family be forgotten. A.P. Carter traveled the Appalachian Mountains, collecting folk songs and then recording them with his family trio. He, together with sister-in-law and June Carter Cash did much to pave the way for modern bluegrass, country and rock and roll.

A folk music revival was launched during the 1960s and 1970s by groups such as the Kingston Trio, who popularized songs long since forgotten. The movement’s popularity later waned, but has emerged today as something generally called Americana. It has that same story-telling underpinning and will ensure that “folk” music continues to be an important part of our musical history.