.mkv File Extension

What kind of file is an .mkv?

An .mkv file is a Matroska multimedia container, usually holding video plus one or more audio or subtitle tracks. It can also store chapters, tags, cover art, fonts, and other attachments. MKV is not a video codec, so two files with the same extension may use completely different video and audio codecs.¹

The registered MIME type is video/matroska. Poly and much existing software also use video/x-matroska, its deprecated alias.² Player compatibility depends on both the container and every track you want to use.

Matroska uses EBML, a binary format with a header element beginning 1A 45 DF A3. Those bytes also begin other EBML documents. A reliable check parses the header, confirms the document type is matroska, and validates the structure rather than trusting the suffix alone.¹³

How do I open an .mkv on my computer?

VLC plays Matroska on Windows, macOS, and Linux and includes its own support for many common codecs.⁴ mpv is another cross-platform choice. MKVToolNix is useful when you need to inspect tracks, edit container metadata, add subtitles, or extract streams rather than simply watch the video.⁵

Built-in players and browsers may open some MKV files but reject others because codec support varies. If video works without sound, inspect the selected audio track and its codec before assuming that the file is damaged.

Poly support for .mkv

Poly gives supported MKV videos a representative thumbnail, timeline previews, in-app playback, technical metadata, visual search, and searchable speech transcripts. Matches can jump to a relevant moment. The Poly agent can use supported visual content, speech, and metadata when answering questions.

Transcription requires usable audio with audible speech. Silent video can still benefit from visual search. Playback and content-aware features depend on whether the streams inside the container can be decoded. See Supported File Types for the video feature overview.

History of .mkv

The Matroska project began on December 6, 2002, after a fork of the Multimedia Container Format project. Its name refers to matryoshka nesting dolls, reflecting how one container can hold many kinds of tracks and related data.¹⁶

In October 2024, the IETF published RFC 9559 as the Standards Track specification for Matroska. It registered video/matroska for .mkv and marked video/x-matroska as a deprecated alias.¹²

Working with .mkv and alternatives

Choose the container that fits the destination:

Remuxing moves compatible encoded tracks into a different container without recompressing them, so it is fast and does not reduce picture quality. Transcoding decodes and re-encodes a track. It can improve compatibility, but lossy encoding may reduce quality. Check every audio track, subtitle, chapter, and attachment before deleting the original.