The RT-11 OS for the DEC PDP used the forward slash for options and didn't have the concept of directories. The filename format for RT-11 was a three character

The RT-11 OS for the DEC PDP used the forward slash for options and didn't have the concept of directories. The filename format for RT-11 was a three character device specifier, 6 character file name and 3 character extension CP/M borrowed most of its structure from RT-11 and also didn't have directories, just devices. The filename format was 8.3 instead of 6.3. MS-DOS in 2.0 (which introduced directories) defaulted to using the forward slash as the command options separator, but had an option in CONFIG.SYS for SWITCHAR that if you set to something other than forward slash, allowed you to use the forward slash as a directory separator.