MKV — Matroska — is the open, flexible video container popular for high-quality private libraries. Where MP4 dominates web streaming and ProRes/DNxHR dominate production, MKV occupies its own niche: enthusiast and archival collections where flexibility and quality matter more than universal device support. The format was designed in 2002 specifically to be feature-rich and codec-agnostic — it can hold virtually any video, audio or subtitle codec, multiple of each, plus metadata like chapter markers, attachments and tags.
The "Matroska" name refers to the Russian nesting doll because the format works in a similar way — boxes inside boxes inside boxes, infinitely extensible. Practically, that means MKV files often carry features that MP4 doesn't: multiple audio tracks (commentaries, alternate languages, descriptive audio), multiple subtitle tracks (including soft subs you can toggle on and off), chapter markers, embedded fonts for stylised subtitles, even attached image files. For a movie collector ripping their Blu-ray collection, MKV preserves everything; an MP4 conversion would have to flatten or drop some of it.
For a product team, MKV is primarily an ingest concern, not a delivery one. You'll encounter MKV files from users uploading content, especially from Linux-heavy or enthusiast audiences. iOS Safari and Apple TV don't play MKV natively — that alone disqualifies it from delivery for any iOS-supporting service. The practical pipeline: accept MKV on ingest, transcode (or transmux if codec-compatible) into MP4 / fragmented MP4 for delivery. Most cloud services and FFmpeg handle MKV input transparently. Its successor is WebM, an MKV-based container restricted to royalty-free codecs (VP9/AV1 + Opus), but MKV itself remains the dominant format for offline enthusiast video.
