One of the three standard platforms (or operating systems) for computer systems—from mainframes to microcomputers—developed in the early 1970s by AT&T's Bell Labs. UNIX was written in the C language, and was the first operating system to allow multitasking, or the simultaneous running of separate applications. UNIX is also used on computers linked to a network and a mainframe, such as university computers. It is also a useful operating system for the development of other applications. UNIX is a popular operating system, primarily for engineering and manufacturing firms, as well as academic institutions. There are also several different types of the UNIX system, which were developed in different places, and the different versions are rarely completely compatible with each other. The Internet iself was originally designed for computers running UNIX. (See Platform.) Varieties of UNIX include both CLI- and GUI-based interfaces.