1. Elizabeth Cotten

Elizabeth "Libba" Cotten wrote her enduring classic "Freight Train" (watch above) at age 12, after saving up money for a guitar by stoking a neighbor's iron stove for $1 a day. A southpaw performing in the Piedmont folk-blues style, Cotten taught herself to play by flipping a right-handed guitar upside-down, picking out bass lines with her index finger while using her thumb for melodies, in a style that bore her name - Cotten picking - and would become widely known and emulated. After giving up the guitar for 25 years (she cared for the children of Ruth Crawford Seeger and Charles Seeger), Cotten resumed playing at 68 and began her long-delayed recording career on Folkways Records. She played until shortly before her death in 1987.

2. Maybelle Carter

"Mother" Maybelle Carter is responsible for creating the concept of a lead guitarist. A founding member of the pioneering Carter Family in the late 1920s, she developed the Carter scratch - using her thumb to pick bass notes and her index finger for rhythmic strums and melody lines - to fill out the trio's sound. Before Carter, the guitar was thought of as simply a tool for background rhythm, not the focal point. By the end of the '20s, writes historians Charles K. Wolfe and Ted Olson in their book The Bristol Sessions: Writings About the Big Bang of Country Music, the Carter scratch "was the most widely imitated guitar style in music. Nobody did as much to popularize the guitar, because from the beginning, her playing was distinctive as any voice.'"

3. Sister Rosetta Tharpe

At a time when mainstream female guitarists were rare, Sister Rosetta Tharpe was a pop star of the '30s and '40s. Tharpe started performing at age four, billed as "the singing and guitar playing miracle," and accompanied her mandolin-playing mother at church and at tent revivals in Arkansas. By 23, she was signed to Decca Records, producing solo and orchestra-backed acoustic albums on her Gibson L-5 and National archtop and steel guitars. Though she went electric in 1947, her early acoustic work - blending gospel with the blues and jazz she discovered after her family moved to Chicago - set the template for what would become modern rock 'n' roll and R&B.

4. Alice Gerrard

In Alice Gerrard's 50-plus year career, she has been a tireless crusader for Americana, Appalachian, and mountain music. The success of her collaboration with Hazel Dickens in the '60s and '70s proved women were a force to be reckoned with in the bluegrass scene at a time when men dominated the genre. The duo performed songs about the struggles of everyday life, particularly for women. "I gravitate toward the darker material in traditional music, and I guess in my own songs too," the 82-year-old told NPR. "I've never had much success writing a funny song, or anything like that. The high, lonesome sound is what appeals to me."

5. Joni Mitchell

"When I'm playing the guitar, I hear it as an orchestra: the top three strings being my horn section, the bottom three being cello, viola, and the bass being indicated but not rooted yet," Joni Mitchell told AG in 1996, of her distinctive guitar style. The fiercely independent artist burst into the American consciousness during the '60s, "battling male egos" and a cutthroat music industry, and has continued to defend her musical reputation, inspire millions, and expand the guitar's sonic possibilities. Mitchell's guitar playing spans a dizzying array of more than 50 alternate tunings, which she uniquely categorizes based on the number of half steps between the notes of adjacent strings.

6. Joan Baez

"Queen of Folk" Joan Baez, singer-songwriter, activist, and guitarist often performs with only a Martin to accompany her sterling soprano. Baez made her major debut at the 1959 Newport Folk Festival, and every day since has been at the forefront of the fight to create social change throughout the United States, from Vietnam to prison reform to LGBTQ equality. "People say music changes the world but the guitar, of all the instruments, is played everywhere," Baez says in a Craft in America documentary. "They've served their purpose of uniting people." Next month, Baez will be inducted into the Rock & Roll Hall of Fame.