These releases can only run one application, except for desk accessories, at a time, though special application shells such as MultiMac[5] or Switcher (discussed under MultiFinder) could work around this. System 1.0, 1.1, and 2.0 use a flat file system called Macintosh File System (MFS). The Finder provides virtual folders that could be used to organize files, but these are not visible from any other application and do not actually exist in the file system.