Unix was developed in 1969 by Ken Thompson and Dennis Ritchie at AT&T, who scaled down the sophisticated, multiuser MULTICS operating system for Digital's PDP-7. The Unix name was coined as a single-processor version of MULTICS (un meaning "one" and ix from the "ics" in MULTICS). By 1974, Unix had matured into an efficient operating system mostly on PDP machines and became popular in scientific and academic environments. Berkeley Was a Major Contributor Considerable enhancements were made to Unix at the University of California at Berkeley, and versions of Unix with the "Berkeley extensions" became widely used. By the late 1970s, commercial versions became available, such as IS/1 and XENIX. Consolidation and Bouncing Around In the early 1980s, AT&T combined the many Unix versions into System III, IV and V (System 3, 4 and 5). During this time, AT&T licensed Unix to universities and other organizations, but was prohibited from marketing it. After Divestiture (1984), it was able to change course, and by 1989 formed the UNIX Software Operation (USO) division. USO introduced System V Release 4.0 (SVR4), incorporating XENIX, SunOS, Berkeley 4.3BSD and System V into one operating system defined by the System V Interface Definition (SVID). In 1990, AT&T spun off USO into UNIX System Laboratories, Inc. (USL). In 1993, Novell acquired the System V source code from USL and sold it to The Santa Cruz Operation (SCO) a year later. At the same time, Novell transferred the UNIX trademark and the specification that later became the Single UNIX Specification to X/Open, which later became The Open Group. See Unix.