PL/1 grew out of a project in the early 1960's to develop a successor to Fortran. It was decided very early on that it would do decimal arithmetic (like COBOL) and then that it would provide good support for record-oriented I/O and the common access methods used for sequential and random-access disk, tape, cards, printers, etc. typically attached to mainframe computers, which you pretty much needed to compile it anyway. This made it quite a horse of a different feather than Algol. Minicomputer and microcomputer versions of PL/1 subsequently appeared (I recall Digital Research had one that implemented something called subset G,), but these were limited subsets, pretty much matching simple implementations of Fortran, Pascal, or Modula-2 feature for feature -- not as elegant as Algol, but about as close to Algol as they were to IBM's full PL/1 level F. For a long time, many IBM System/360 and System/370 mainframe were not suitable to run the famous IBM OS/360, and there were two levels of simplified versions of the operating system (called DOS and DOS/VSE), and each of these, in turn, offered only its own distinct subset version of the full mainframe PL/1.