This is not true. Most mainframe OSes have flat filesystems which do not have the concept of subdirectories. They also do not use command switches. The behav

This is not true. Most mainframe OSes have flat filesystems which do not have the concept of subdirectories. They also do not use command switches. The behaviour was almost certainly inherited from DEC TOPS-10: http://www.os2museum.com/wp/why-does-windows-really-use-backslash-as-path-separator/ http://www.os2museum.com/wp/why-does-windows-really-use-back... This is not a mainframe OS, or an IBM one either. It ran on a 36-bit DEC minicomputer, the PDP-10.