Blended voices — when folkies collaborate

One of the virtues of folk-oriented music is how comfortably it lends itself to artistic collaboration. I often buy recordings by artists whose work I don’t know if I learn that they have recorded with someone I enjoy and respect. Joan Baez introduced a variety of artists to her audience with the Ring Them Bells album. I’ve learned about a number of musicians over the years by reading the liner notes on Emmylou Harris’s CDs. My list of collaborations would be very long indeed (not only recordings but live concerts). What are some of your favorite folkie/acoustic/rootsie collaborations?