A file with .x extension refers to DirectX 3D Graphics legacy file format that was introduced with Microsoft DirectX 2.0. It was used for 3D graphics rendering in games and specifies the structures for meshes, textures, animations, and user-defined objects. It has been deprecated since 2014 as the Autodesk FBX file format serves better as a more modern format. X is template-driven and is free of any usage knowledge.
You can open DirectX X files using Microsoft DirectX and common text editors.
X File Format
The X file reference contains reference information for the API elements that are used to work with DirectX .x files. The format provides low-level data primitives that are used by other applications to define higher-level primitives through data templates. DirectX 6.0 introduced interfaces and methods that enable reading from and writing to .x files. DirectX 3.0 introduced a binary version of this file format.
The X file format reference defined by DirectX 9 contains reference information for .x files in Binary as well as Text Encodings.
The binary format defines the DirectX X format as a tokenized representation of the text format. These tokens can be standalone to give grammatical structure or can be record-bearing tokens supplying the necessary data.
The binary header can be read and write using the following definitions.