FourCCs

FourCCs, short for four-character codes, are compact identifiers used in multimedia files to label the type of data or the codec used to encode it. A FourCC is typically a sequence of exactly four ASCII characters and is stored as a 32-bit value in the file, often with little-endian byte order in practice. In many containers, including RIFF-based formats like AVI and in QuickTime / MP4 family structures, the FourCC guides the decoder on how to interpret the following data.

Originating in the early 1990s with Video for Windows from Microsoft and IBM, FourCCs were designed to identify codecs and data blocks within AVI files. Over time they have been adopted by other container formats and remain a common mechanism for signaling how to parse a stream.

Common examples include DIVX (DivX), XVID (Xvid), MP4V (MPEG-4 Visual), MJPG (Motion JPEG), and H264 or AVC1 (H.264 video in various containers). Note that the same four-character code may occur in different containers with different meanings, and some containers use slightly different spellings or case conventions.

Limitations include the lack of a universal, centralized registry; FourCCs are not guaranteed to be unique across all formats, can change meaning between containers, and rely on implementation conventions. They are primarily identifiers, not comprehensive metadata about capabilities or parameters.

In practice, software such as media players, editors, and codecs read FourCCs to select the appropriate decoding path or bitstream handling. They remain a lightweight, historically rooted method for tagging media streams despite newer container technologies.