Windows Preceder is a term used to refer to the operating systems that preceded Windows. The two most notable of these are MS-DOS (5 letters) and DOS (3 letters). MS-DOS was developed by Microsoft in 1981 and was the first widely used operating system for personal computers. DOS, or Disk Operating System, was developed by IBM in 1981 and was the first operating system to be used on IBM PCs. Both MS-DOS and DOS were popular operating systems until the release of Windows in 1985.