Why Microsoft Stuck with Backslash \ Instead of Forward Slash / for 40+ Years

Developers notice that macOS and Linux use / while Windows uses \ as the path separator, a difference that has persisted for over 40 years.

The article traces the origin to Unix, where Ken Thompson and Dennis Ritchie chose / in the early 1970s because it was readily available on the keyboard and caused no conflicts.

DEC's TOPS‑10 used / as a command‑line option prefix, a habit that many early engineers carried to IBM.

When Tim Paterson wrote 86‑DOS (later MS‑DOS) in 1980, IBM engineers who wrote the DOS utilities inherited the / prefix for options such as DIR /W, FORMAT /Q, and COPY /Y. At that time DOS had no sub‑directories, so / was never a path separator.

During the 10‑month development of DOS 2.0 (1982‑early 1983), Microsoft added a hierarchical file system and needed a directory separator. Microsoft engineers naturally wanted to reuse /, but IBM objected because changing the meaning of / would break millions of existing DOS programs that already used it as an option prefix.

IBM, as the dominant customer, forced Microsoft to find an unused character on the keyboard. The only remaining candidate was the backslash \, which was then adopted as the Windows path separator.

Since Windows NT (1993) the Win32 API has accepted forward slashes, but the command shell, PowerShell, Explorer and countless scripts still rely on \. The article concludes that the backslash is a legacy compatibility compromise rather than a technical necessity, and that the “debt” remains unresolved.