As the word loop suggests, the use of loops in electronic music dates back to the earliest days of tape music. The first electronic music studio dedicated primarily to the use of tape techniques was opened in Paris in 1948 by Pierre Schaeffer. He and Pierre Henri, who joined a year later, pioneered many of the tape techniques that are the forerunners of sample-editing and digital-audio-sequencing techniques used today. Primarily, they recorded everyday sounds on tape and then cut and spliced them to form collages—a style they called musique concrète.
Guitarist Les Paul, another early experimenter with tape techniques, is credited with inventing multitrack recording, tape echo, and sound-on-sound in the 1950s. Minimalist composers made extensive use of tape techniques beginning in the 1960s. In particular, Steve Reich's two speech-loop tape pieces It's Gonna Rain and Come Out introduced slowly shifting phase relationships between two tape loops of different lengths. John Lennon's Revolution 9 (1968) is almost entirely a collage of clips and loops of sounds from the EMI sound archive. From the 1950s to the late 1980s, tape techniques played a major roll in virtually all styles of music.
In the world of tape there are only a few basic strategies. You can record sound and play it back. You can cut the tape up and splice the pieces together in a different order. You can play the tape backwards by reversing the reels and flipping them over. You can vary the speed. (In the early days, speed could vary between two settings; now speed can vary continuously.) You can dub the playback of one or more tape players onto another tape recorder. With multitrack recorders you can also dub some tracks to others on the same machine. And finally, you can play loops on one or more tape players. That opens the door to flanging as well as the shifting phase techniques just mentioned. All of those tape techniques can be replicated in digital-audio-sequencing and sample-editing software.