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concepts / containers

What is Container Format?

Every file is a box
with boxes inside.

The extension names the container: labelled blocks that say what's inside and where. An MP4 holds a video stream and an index; a DOCX is a ZIP of XML files. Pick one, or open your own, and see it laid out byte by byte.

upload_fileOpen your own file

description.docx

Word documentthree XML files in a ZIP

graphic_eq.wav

WAV soundformat, tags, then samples

image.png

PNG imagechunks, each with a checksum

photo.webp

WebP imageRIFF again, holding a VP8 picture

videocam.webm

Recorded videoyour browser records 1.5 s

ZIP · Office Open XML (DOCX)letter.docx

payloadindexheadermetadatapadding / other

top

[Content_Types].xml_rels/.relsword/document.xmlcentral directory

byte 01.1 KB

[Content_Types].xmlpayload

stored, 297 B unpacked

offset0

size346 B

of file31.7%

first 32 bytes of this block

00000050 4b 03 04 14 00 00 00PK······

00000800 00 00 00 21 5a 91 cf····!Z··

0000101f 05 29 01 00 00 29 01··)···)·

00001800 00 13 00 00 00 5b 43······[C

Boxes inside boxes

5 top-level blocks · 5 in total

[Content_Types].xml346 B

stored, 297 B unpacked

_rels/.rels302 B

stored, 261 B unpacked

word/document.xml238 B

stored, 191 B unpacked

central directory185 B

the index of every entry, at the end

end record22 B

points to the directory

Which codecs fit in which box

"Allowed" is what the container's specification permits. "Plays here" asks this browser, live, whether it would play that exact combination. A file can be perfectly valid and still not play, because the box is supported but the codec inside isn't.

allowed, plays hereallowed, not herenot allowed

H.264HEVCAV1VP9AACOpusFLACMP3

MP4video/mp4✓·✓✓✓✓✓✓

MOVvideo/quicktime····

MKVvideo/x-matroska✓·✓✓✓✓✓✓

WebMvideo/webm✓✓✓

Oggvideo/ogg✓✓

Recognise a box by its first bytes

ISO base media2001

…·· ftyp (bytes 4-7)MP4, M4A, MOV, 3GP, HEIC, AVIF, CR3

RIFF1991

RIFF ···· + form typeWAV, AVI, WebP, ANI

EBML (Matroska)2002

1A 45 DF A3MKV, WebM, MKA

ZIP1989

50 4B 03 04DOCX, XLSX, PPTX, EPUB, ODT, JAR, APK

Ogg1993

OggSOGG, OPUS, OGV

PNG chunks1996

89 50 4E 47 0D 0A 1A 0APNG, APNG

JPEG segments1992

FF D8JPG, and the picture inside many other files

PDF1993

%PDF-PDF, AI (recent Illustrator files)

movieWhat a codec isThe thing inside the box: how video and audio are squeezed.MP4 → H.264 + AAC swap_horizVideo converterChange the box only (remux) or the codec too.clip.mkv → clip.mp4 folder_zipZIP formatEvery header in a ZIP, local and central.photos.zip → entries · directory searchFormats explainedIdentify any file from its first bytes.mystery.bin → it’s a WebP

Frequently Asked Questions

What is the difference between MP4 and H.264?

MP4 is the container (wrapper). H.264 is the video codec (compression method) inside the container. MP4 + H.264 is the most common combination.

Does changing the container affect quality?

No. Changing only the container (remuxing) without re-encoding the video data has zero quality loss.

Can I change MKV to MP4 without quality loss?

Yes. MKV to MP4 conversion is usually just a container change - the video data is identical. No re-encoding needed.

What container does YouTube use?

YouTube accepts many containers but recommends MP4. Internally, YouTube uses its own streaming formats based on MP4 and WebM.

What is MKV vs MP4?

Both are video containers that can hold H.264 video. MKV supports more features (multiple audio tracks, subtitles). MP4 has better compatibility.