The IBM MT/ST (Magnetic Tape/Selectric Typewriter, and known in Europe as MT72) was a model of the IBM Selectric typewriter, built into its own desk, integrated with magnetic tape recording and playback facilities, located in an attached enclosure, with controls and a bank of relays. It was released by IBM in 1964. It recorded text typed on 1/2" perforated magnetic tape, approximately 25 kilobytes per tape cassette, and allowed editing and re-recording during playback. It was the first system marketed as a word processor. Most models had two tape drives, which vastly facilitated revision and enabled features such as mail merge. An add-on module added a third tape station, to record the combined output of playback from the two stations.
The MT/ST automated word wrap, but it had no screen, automated hyphenation (soft hyphens were available), or concept of the page; pages had to be divided and numbered by the human operator during playback. Instruction manuals taught the operator the importance of listening to the sounds of the machine during playback. The backspace key backed up the tape so a character could be recorded over; there was also a true backspace code, which allowed overstruck characters, like á. Insertion capabilities were limited: one could insert while copying from one tape station to the other; on a single tape one null character per line was reserved for insertions. A "switch code" instructed the playback to switch to the other tape drive. In a cumbersome way, points on the tape could be marked and jumped to.
The MT/ST was not electronic; it implemented its functions through a bank of electromechanical relays.
The MT/ ST was followed by the Magnetic Tape Selectric Composer, which differed only in output device, and for which it served as input. (Tapes prepared or edited on the MT/ST could be played back on the MT/SC.) Thus automated line justification, with three type sizes (10, 12, and 15 characters per inch), bold, italic, and a variety of type styles (implemented using type "balls" similar to those of the Selectric typewriter) was made available within the budget of a medium-sized office or publisher. When printing a document, the machine would pause printing at the stop code, wait for an operator to change the Selectric type ball and press a button telling the device to continue.
The MT/ST was also used as a data entry device for early issues of the RILM Abstracts scholarly publication at the City University of New York Graduate Center. Cartridges created on the MT/ST were read by an IBM 2495 Tape Cartridge Reader onto an IBM 360 mainframe for further processing before being sent to be printed.
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