Why does Windows use backslashes for paths and Unix forward slashes?

Unix introduced / as the directory separator sometime around 1970. I don't know why exactly this character was chosen; the ancestor system Multics used >, but the designers of Unix had already used > together with < for redirection in the shell (see Why is the root directory denoted by a / sign?).

MS-DOS 2.0 introduced \ as the directory separator in the early 1980s. The reason / was not used is that MS-DOS 1.0 (which did not support directories at all) was already using / to introduce command-line options. It took this usage of / from CP/M, which took it from VMS. You can read a more thorough explanation of why that choice was made on Larry Osterman's blog (MS-DOS even briefly had an option to change the option character to - and the directory separator to /, but it didn't stick).

/ it is recognized by most programmer-level APIs (in all versions of DOS and Windows). So you can often, but not always get away with using / as a directory separator under Windows. A notable exception is that you can't use / as a separator after the \\? prefix which (even in Windows 7) is the only way to specify a path using Unicode or containing more than 260 characters.

Some user interface elements support / as a directory separator under Windows, but not all. Some programs just pass filenames through to the underlying API, so they support / and \ indifferently. In the command interpreter (in command.com or cmd), you can use / in many cases, but not always; this is partly dependent on the version of Windows (for example, cd /windows works in XP and 7 but did not in Windows 9x). The Explorer path entry box accepts / (at least from XP up; probably because it also accepts URLs). On the other hand, the standard file open dialog rejects slashes.

The underlying Windows API can accept either the backslash or slash to separate directory and file components of a path, but the Microsoft convention is to use a backslash, and APIs that return paths put backslash in.

MS-DOS 2.0 copied the hierarchical file system from Unix and thus used the forward slash, but (possibly on the insistence of IBM) added the backslash to allow paths to be typed into the command shell while retaining compatibility with MS-DOS 1.0 and CP/M where the slash was the command-line option indicator.

• Compare dir/w, which shows the current directory in wide format, against dir\w, which runs the file w within the dir directory.

References:

• Wikipedia: Backslash
• Larry Osterman's WebLog