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Why does Windows use backslash as path separator? (2019)

More or less anyone using modern PCs has to wonder: Why does Windows use backslash as a path separator when the rest of the world uses forward slash? The clear intermediate answer is “because DOS and OS/2 used backslash”. Both Windows 9x and NT were directly or indirectly derived from DOS and OS/2, and certainly inherited much of the DOS cultural landscape.

When DOS 2.0 added support for hierarchical directory structure, it was more than a little influenced by UNIX (or perhaps more specifically XENIX), and using the forward slash as a path separator would have been the logical choice. In retrospect it seems silly to change the path separator in order to preserve backward compatibility with a short-lived early PC operating system, but hindsight is 20/20 and in 1982, the future popularity of UNIX and DOS (and its derivatives) was incredibly non-obvious. Microsoft’s tools for the 8080(F80 FORTRAN compiler, M80 macro assembler, LINK 8080 linker all used the forward slash as a switch character separating command line options, and did so at least as far back as 1977.

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