PL/I (Programming Language One) is an old language developed for use on IBM Mainframes. Its primary function is Data processing. Until the time this new language was developed, all previous languages had focused on one particular area of application, such as science, artificial intelligence, or business. PL/1 was not designed to be used in the same way. It was the first large-scale attempt to design a language that could be used in a variety of application areas.