Forward slash was supported by the DOS API starting with version 2 of DOS, as previous versions of DOS did not support hierarchical directories (as Keith noted in a subsequent post). IBMs desire to have forward slash serve as the command line parameter switch character starting with DOS 1.whatever meant that you could type things like: ``` A>DIR/W ``` And it would be recognized as a DIR command followed by no white space and command arguments. As the customer, IBM did not want DOS 2 to break that convention that many people had some to expect, and allowing forward slash as a path separator would add ambiguity to parsing of the command line: does `DIR/W` mean show a wide directory or does it mean run the W program in the subdirectory named DIR? So yes, the engineers at MS knew of the convention that forward slash was a path separator, but they were enhancing a system based on CP/M conventions and IBM imposed requirements. It's always been an issue from the command line, but never from the API from the time DOS began supporting hierarchical directories.