FourCC

FourCC, short for Four Character Code, is a sequence of four ASCII characters used to identify the data format or codec of a multimedia stream within a container file. It serves as a compact, human-readable identifier that helps software determine how to decode or process a stream without inspecting the entire bitstream. The term is widely used in video and audio containers and has become a de facto standard in many formats.

In RIFF-based containers such as AVI, the FourCC is stored in headers to indicate the codec used for video or audio streams. In modern formats like QuickTime and MP4, four-character codes such as avc1 (for certain AVC/H.264 video), hev1 (another video tag), and mp4a (AAC audio) identify the track’s codec. FourCC-like values are also used in other APIs and platforms, for example the Pixel Format FourCCs in Video4Linux such as YUYV and NV12. Because the on-disk representation may be stored in little-endian order, the four characters displayed when reading the 32-bit value may require byte-order adjustment to obtain the intended string.

FourCC codes are not a formal ISO standard; they are a de facto convention with mappings that vary by container format and vendor. They remain a convenient, human-readable shorthand for codec identification, even as metadata schemes have grown more expressive.

Examples include DIVX, XVID, MJPG for certain video streams and AVC1, MP4A, HEV1 for others. The choice of FourCC can affect compatibility and decoding capabilities across players and devices.