Arc is a composition of feedback, guitar noise, and vocal fragments that was realized from various shows on the 1991 Neil Young and Crazy Horse US tour, which was originally released with Weld in a special-edition 3-CD set called Arc-Weld.
According to an interview with Steve Martin of Agnostic Front that appeared in the December 1991 issue of Pulse! magazine, Arc had its genesis in a film that Young made called Muddy Track (referred to in an interview with David Fricke in the November 28, 1991, issue of Rolling Stone), which consisted of the beginnings and endings of various songs from his 1987 European tour. Young placed a video camera on his amplifier during the 1987 tour and recorded the beginnings and endings of various songs, and later edited them down into the film's soundtrack. "It was the sound of the entire band being sucked into this little limiter, being compressed and fuckin' distorted to hell," Young said to Martin, referring to the soundtrack of Muddy Track. Young then showed the video to Sonic Youth's Thurston Moore, who suggested that he record an entire album in a similar manner. However, Arc was not recorded through video camera microphones, as was the case with Muddy Track, but instead was compiled from various professional multi-track recordings made throughout the tour.