MKV

The single most useful thing to know about MKV is that it does not describe how the picture is compressed. That job belongs to the codec — H.264, HEVC, AV1 — and the same codec can sit in an MP4, an MKV or a WebM without a single bit of the video changing. The container decides how the pieces are labelled, ordered and found; the codec decides what the pieces are.

What Matroska is unusually good at is holding a lot of pieces at once. One file can carry the film, an original audio track, a dubbed one, a commentary, eight subtitle languages, chapter marks, cover art and the font files needed to draw the subtitles the way their author intended. That is why it became the default for anything archival: a Blu-ray can be moved into one MKV without throwing away the parts that make it a Blu-ray.

It costs nothing to use, has no patent pool behind it and has been a stable, openly published specification for over twenty years. None of which persuaded Apple to support it.

Apple's media stack reads the containers Apple ships support for: MP4, MOV, M4V, 3GP and a handful of audio wrappers. Matroska is not among them and never has been. There is no setting to change and no codec to install, because the missing piece is not a codec — the system simply has no code that knows how a Matroska file is laid out.

So the failure happens before any decoding starts. The Files app shows the file, the preview refuses it, and on a Mac QuickTime Player says it is not a movie file. The video inside might be plain H.264 that the very same device decodes in hardware all day long. It never gets that far.

This is also why converting an MKV is so often near-instant. If the streams inside are already ones MP4 accepts, nothing needs re-encoding: the pieces are lifted out and re-labelled into a new container. That operation is called remuxing, and it runs at disk speed. A conversion that takes half an hour is one that re-encoded video which did not need it.