The Direct X software development kit (SDK) consists of runtime libraries in redistributable binary form, along with accompanying documentation and headers for use in coding.
Originally, the runtimes were only installed by games or explicitly by the user.
The API was developed jointly between Microsoft and Nvidia, which developed the custom graphics hardware used by the original Xbox.
DOS allowed direct access to video cards, keyboards, mice, sound devices, and all other parts of the system, while Windows 95 – with its protected memory model – restricted access to all of these, working on a much more standardized model.
Eisler wrote about the frenzy to build Direct X 1 through 5 in his blog.
Direct X 2.0 became a component of Windows itself with the releases of Windows 95 OSR2 and Windows NT 4.0 in mid-1996.
The name Direct X was coined as a shorthand term for all of these APIs (the X standing in for the particular API names) and soon became the name of the collection.
When Microsoft later set out to develop a gaming console, the X was used as the basis of the name Xbox to indicate that the console was based on Direct X technology.