Reel-to-reel audio tape recording is the form of magnetic tape audio recording in which the medium is held on a reel that is not permanently mounted in an enclosed cassette. In use, the feed reel containing the tape is placed on a spindle or hub, and the end of the tape is manually pulled out of the reel, threaded through mechanical guides and a tape head assembly to the hub of the second, initially empty takeup reel.
The reel-to-reel format was used in the earliest tape recorders, including the pioneering German-British Blattnerphone (1928) machines of the late 1920s which used steel tape, and the German Magnetophon machines of the 1930s. Originally, this format had no name, since all forms of magnetic tape recorders used it.
Reel-to-reel systems use tape that is 1⁄4, 1⁄2, 1, or 2 inches (6.35, 12.70, 25.40, or 50.80 mm) wide, which normally moves at 3+3⁄4, 7+1⁄2, 15 or 30 inches per second (9.5, 19.1, 38.1 or 76.2 cm/s). All standard tape speeds are derived as a binary submultiple of 30 inches per second.
Read more about the history of reel-to-reel audio at Wikipedia.