Synthesizer sounds were added by producer Brian Eno on a Yamaha DX7 keyboard, including a "squishy, mad-synth sound" to create a "dovetail" connecting the different segments of the song. The Edge later added guitar tracks, as well as additional sounds to the song using an EMS Synthi A synthesiser. Once the song was almost finished, the band had doubts about the first half's backing track. Having established an arrangement for the song, they performed it again in the studio and used the new recording for the first half, while using portions of the new performance for the second half.