ISO Base Media File Format

The ISO Base Media File Format (ISOBMFF, ISO/IEC 14496-12) is a generic container structure: a file is a tree of “boxes”, each with a size and a four-character type. It is not something you encode to directly. It is the base that other formats specialise, and that is what the common file extensions actually are.

How the names relate

Name	What it is
ISOBMFF (14496-12)	The base format: boxes, tracks, sample tables, fragments
MP4 (14496-14)	ISOBMFF plus rules for MPEG-4 codecs; .mp4, .m4a, .m4v
MOV / QuickTime	Apple’s original format that ISOBMFF and MP4 were derived from; still differs from MP4
3GP / 3G2	Mobile-phone profiles from 3GPP/3GPP2
HEIF / AVIF / HEIC	Still images (and sequences) stored as items in an ISOBMFF file
CMAF, fragmented MP4	ISOBMFF fragments used for DASH and HLS streaming

An “MP4 file” is an ISOBMFF file whose ftyp brand is isom, mp42 or similar, and a “MOV file” is one with brand qt (padded with two spaces to four characters). Players decide by brand and by what the sample entries contain, not by extension. MOV and MP4 share the box structure but QuickTime allows codecs and track types MP4 does not define, which is why a .mov full of ProRes plays in Apple software but not in a typical MP4 player.

Boxes you will meet

Box	Role
ftyp	Major brand and compatible brands: identifies the profile
moov	Movie header and track descriptions, including the sample tables
mdat	The encoded samples themselves
moof / mfra	Per-fragment index and random-access index in fragmented files
stsd sample entry	Codec identifier such as avc1, hvc1, hev1, av01, mp4a, Opus
colr, mdcv, clli	Colour description, mastering display and content light level (HDR) signalling

Each codec has a binding that defines its sample entry and configuration box, for example av01 with av1C for AV1, avc1 with avcC for H.264, and Opus with dOps for Opus. The Opus binding on opus-codec.org is still a draft but is what encoders and players implement. The MP4 Registration Authority lists registered brands and sample entries; being registered does not mean a particular player supports it.

Codecs and formats documented for ISOBMFF on this wiki

Relationship	Project or format	Evidence and scope	Status
Carried By by	AV1	Uses the av01 sample entry and av1C configuration box.Scope: Container Mapping · Sources: AV1 Codec ISO Media File Format Binding	Normative
Registered With by	Dolby Vision	MP4RA registers dav1 for Dolby Vision carried with AV1; actual support is profile- and vendor-dependent.Scope: Container Mapping · Sources: MP4 Registration Authority codec registry	Registered
Carried By by	Opus	The public Opus binding defines the Opus sample entry and dOps box, but remains an old incomplete draft.Scope: Container Mapping · Sources: Encapsulation of Opus in ISO Base Media File Format, MP4 Registration Authority codec registry	Registered
Carried By by	AVC / H.264	MP4RA registers AVC sample entries including avc1 and avc3.Scope: Container Mapping · Sources: MP4 Registration Authority codec registry	Registered
Carried By by	HEVC / H.265	MP4RA registers HEVC sample entries including hvc1 and hev1.Scope: Container Mapping · Sources: MP4 Registration Authority codec registry	Registered
Carried By by	VP9	The WebM Project publishes a VP codec binding for ISO Base Media File Format.Scope: Container Mapping · Sources: VP Codec ISO Media File Format Binding	Documented
Carried By by	VVC / H.266	MP4RA registers VVC sample entries including vvc1 and vvi1.Scope: Container Mapping · Sources: MP4 Registration Authority codec registry	Registered
Operates On by	MP4Box v26.07.0	MP4Box is GPAC's ISO Base Media File Format inspection, muxing, fragmentation, and packaging CLI.Scope: Muxing Fragmentation Packaging · Sources: GPAC v26.07.0 release, MP4 Registration Authority codec registry	Documented

Layout: progressive and fragmented

A conventional file has one moov and one mdat. Putting moov first (-movflags +faststart in FFmpeg) lets a file start playing during download. A fragmented file instead interleaves moof + media chunks, which is what live recording, DASH, CMAF and browser Media Source Extensions need. These are two different layouts; see MP4 for the commands.

Inspecting a file

ffprobe -v error -show_entries format_tags=major_brand,compatible_brands -show_streams input.mp4

ffprobe -v trace prints the box tree. For full box-level inspection or HEIF/AVIF image items, use MP4Box (MP4Box -info); ffprobe reports streams, not boxes or item properties.