Skip to content

Glossary

A small file that carries what the image format can't hold itself

A sidecar file stores metadata beside an image it can't hold itself — same name, a different extension, and readable by any app that knows the format.

· 3 min read

A sidecar file sits next to an image with the same filename and a different extension — beach.CR2 and beach.xmp in the same folder — and stores metadata the original format either can't hold safely or wasn't designed to hold at all: ratings, keywords, captions, and often a record of edits applied in whatever app wrote it.

The name describes the relationship rather than any one file format. The most common kind on a Mac is XMP, Adobe's open standard, but the pattern — a same-named companion file carrying what the original can't — shows up under other extensions too.

Why RAW files need one and JPEGs usually don't

A JPEG or TIFF can often hold XMP metadata inside itself, in a block the image data doesn't touch. A camera RAW file is a different case: it's a record of exactly what the sensor captured, and a lot of software treats that as something you shouldn't risk rewriting, even in a metadata-only section. The sidecar solves that by keeping the original untouched and parking the ratings, keywords and edit history in a second file that any application reading the same standard can pick up.

~/Pictures/Reference
  • RAW imports
  • coii-thumbnailswritten by Coii Ref
  • coii-proxieswritten by Coii Ref
  • coii.dbwritten by Coii Ref
A sidecar is a second file with the same name — move or copy the pair together, or the rating travels without the picture it describes.

The thing that trips people up

Sidecars travel with the image only if whatever you use to move files treats them as a pair. Copy beach.CR2 to a new drive and forget beach.xmp, and the rating, keywords and edit history are simply gone from that copy — nothing is corrupted, the metadata just didn't come along. Most photo software that writes sidecars will move or copy them automatically when you move the image through the app itself; the risk shows up when files get moved outside it, in Finder or on the command line, or when a backup tool is configured to match one file extension and silently skips the other.

Renaming has the same failure mode. A sidecar is matched to its image by filename, not by any id stored inside either file, so renaming beach.CR2 to beach-final.CR2 without renaming beach.xmp to match breaks the pair just as completely as leaving it behind. Anything that renames files in bulk, including some organizing tools, needs to know to carry the sidecar along for the rename to be safe.

Where this shows up in the reference-manager category

Adobe Bridge is the clearest example in this space: it writes ratings and keywords into the file as XMP where the format allows it, and into a sidecar file where it doesn't — the full comparison covers exactly which formats fall on which side of that line. The practical upshot is that metadata written in Bridge is readable by any other application that understands XMP, sidecar included, rather than trapped inside one app's database.

Coii Ref takes a different approach to the same underlying question: it reads EXIF that's already in a file rather than writing new metadata into it or beside it, and keeps ratings, tags and notes in its own workspace database instead. That's a real difference worth knowing before you choose between them, not a detail to gloss over — what actually happens to a library if the app behind it stops being updated is the fuller version of the question a sidecar file is one answer to. For the broader category this term belongs to, what a reference manager actually is is the place to start.

Questions

Why doesn't the metadata just go inside the image file?
Sometimes it does — JPEG and TIFF can hold XMP metadata directly. A sidecar exists for formats that can't be edited safely that way, most commonly RAW files, where writing into the original risks the camera data it holds.
Does deleting a sidecar file delete the image?
No. They're two separate files that happen to share a name. Deleting the sidecar loses the metadata — ratings, keywords, edit history — and leaves the image itself untouched.

Read next