X is the extension given to files associated with the DirectX feature set. It is a graphics support tool used by Microsoft systems. X files are used to store data related to 3D graphic objects. This is plain text data that includes 3D model mesh data (this structure describes the surface of a 3D model most often using polygons), but also information on textures and object animations.

The X development files and the Microsoft DirectX tool are mainly used in the production of video games.