MKVToolNix

MKVToolNix is the reference toolset for Matroska files, the container most commonly seen as MKV. It is a collection of programs rather than one application: mkvmerge builds and merges files, mkvextract takes tracks out, mkvpropedit edits a file's headers, mkvinfo reports what a file contains, and the MKVToolNix GUI puts all of them behind a graphical interface. The author publishes it under the GNU General Public License v2.

Lossless work on containers, not on pictures

The important thing to understand is that these tools never re-encode. Video and audio streams are copied byte for byte, so a merge or a split takes seconds and cannot degrade quality. What the tools change is everything around the streams: the order of tracks, their languages, names and flags, chapter entries, tags, attached files such as fonts, and the container's own metadata. Menus, chapters, subtitles in SRT, ASS, SSA, PGS and VobSub form, and audio in almost any common codec can all be carried across or converted between subtitle text formats. WebM output is supported for the subset of codecs that format allows.

Working inside the graphical toolbox

The multiplexer window is where most jobs happen: add a source, tick the tracks to keep, set their language and default flags, attach chapters, and write the result. The chapter editor works with both simple and ordered chapter formats, the header editor exposes properties that can be changed in place without rewriting the whole file, and the info tool answers the question of what a file actually contains when a player refuses to open it. Jobs can be queued and run later, which is handy when building a library overnight, and every GUI action has a command-line equivalent for scripting.

Practical configuration and limits

Both 32-bit and 64-bit Windows installers are published, with portable archives alongside them. Because nothing is re-encoded, a merge needs enough free space for the complete output file, and appending two files only produces smooth playback when their codec parameters match. Splitting is done at keyframes or by size and duration, so the split points may not land exactly on the time you asked for. mkvpropedit is fast but also unforgiving: it edits files in place, so keep a copy when experimenting.

Who should choose something else

If a track needs to be converted rather than copied, you need an encoder such as HandBrake or ffmpeg, because MKVToolNix deliberately has none. For cutting at arbitrary frames, a video editor is more appropriate than a keyframe-based split. And for MP4 work the MP4Box or ffmpeg toolchains are more idiomatic, though mkvmerge reads and writes several non-Matroska inputs quite happily.