MKVToolNix is a set of tools to create, alter and inspect Matroska files
Matroska is an extensible, open source, open standard Multimedia container.
Matroska is usually found as .MKV files (matroska video), .MKA files (matroska
audio) and .MKS files (subtitles) and .MK3D files (stereoscopic/3D video).
It is also the basis for .webm (WebM) files.