Tip #539: What is a Sidecar File?

… for Codecs & Media

Tip #539: What is a Sidecar File?

Larry Jordan – LarryJordan.com

Sidecar files track data that the main image file can’t.

Image courtesy of Pexels.com.
Sidecars hold stuff the main file can’t.

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Sidecar files are XML computer files that store data (often metadata) which is not supported by the format of a source file. There may be one or more sidecar files for each source file.

In most cases the relationship between the source file and the sidecar file is based on the file name; sidecar files have the same base name as the source file, but with a different extension. The problem with this system is that most operating systems and file managers have no knowledge of these relationships, and might allow the user to rename or move one of the files thereby breaking the relationship.

Examples include:

  • XMP. Stores image metadata.
  • THM. Stores digital camera thumbnails
  • EXIF. Stores camera data to keep it from becoming lost when editing JPG images.


Rather than storing data separately, it can be stored as part of the main file. This is particularly done for container files, which allow certain types of data to be stored in them. Instead of separate files on the file system, multiple files can be combined into an archive file, which keeps them together, but requires that software processes the archive file, rather than individual files. This is a generic solution, as archive files can contain arbitrary files from the file system.

Container formats include QuickTime, MXF and IFF.

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