Glossary
What is H.264?
By the KeyMo team · Updated
H.264, also called AVC or Advanced Video Coding, is a video compression standard that squeezes video into much smaller files so it can be uploaded, streamed and played. It describes the video inside a file, not the file type. YouTube recommends H.264 for uploads, and KeyMo exports its clips as H.264 video with AAC audio.
H.264 definition
The International Telecommunication Union publishes H.264 under the title Advanced video coding for generic audiovisual services, and its page lists the 06/26 edition as in force. A codec like this is a recipe for compressing and decompressing video. The encoder shrinks the pictures, and any player that knows the recipe can rebuild them.
You will see the same thing called AVC, MPEG-4 AVC or simply H.264. In ordinary use they mean one standard. For the general idea of a codec, see the codec page.
H.264 is not the same as MP4
People say "H.264 file" when they mean an MP4. The two sit at different levels, and mixing them up is the commonest source of confusion.
| Term | What it is | Example |
|---|---|---|
| Video codec | How the pictures are compressed | H.264, or HEVC (also called H.265) |
| Audio codec | How the sound is compressed | AAC |
| Container | The file that holds the video, audio and timing together | MP4, MOV |
The MP4 file format is defined by ISO/IEC 14496-14. Apple describes the QuickTime file format, used for MOV, as a distinct file type built on atoms that describe the movie data. An MP4 or MOV can hold H.264, but the container does not decide the codec. See MP4 for the container side.
What the platforms ask for
| Platform | Codec it names | Notes |
|---|---|---|
| YouTube | H.264 recommended | MP4 container, High Profile, closed GOP, 4:2:0 chroma; HEVC (H.265) is also on its accepted list |
| Instagram (API publishing) | HEVC or H264 | Progressive scan, closed GOP, 4:2:0, AAC audio |
| Instagram Reels ads | H.264 | Plus MP4 or MOV, 9:16, stereo AAC audio |
| TikTok organic uploads | Not stated | The pages read list file types (.mp4, .mov) for ads and name no codec |
Treat these as the platforms' stated preferences. Where a page gives no codec, as with TikTok, we do not guess one.
H.264 or H.265?
H.265, also called HEVC, is a different standard. YouTube accepts HEVC uploads and the Instagram publishing docs list HEVC next to H264. The pages read give no size or quality comparison, so we make none.
Decision rule: when a platform asks for H.264 or lets you pick, choose H.264 and move on. Switch only if a platform's own page tells you to.
Common mistakes
- Renaming a file to .mp4 and expecting the codec to change. Only re-encoding does that.
- Using the audio codec as a clue to the video codec. AAC audio can sit with several video types.
- Re-exporting over and over. Each pass recompresses what the last pass produced.
- Adding black bars to fit a platform. YouTube advises against padding inside the video.
Where KeyMo touches it
KeyMo clips come out as H.264 video with AAC audio, 1080x1920 and 30 fps (see frame rate). KeyMo accepts MP4 and MOV uploads.
Sources: ITU-T: Recommendation H.264, Advanced video coding (checked 11 October 2026); YouTube Help: Recommended upload encoding settings (checked 11 October 2026); YouTube Help: Supported file formats (checked 11 October 2026); Instagram Platform: Content publishing API reference (Reels) (checked 11 October 2026); Meta Ads Guide: Instagram Reels video ads (checked 11 October 2026); ISO: ISO/IEC 14496-14:2020, MP4 file format (checked 11 October 2026); Apple Developer: QuickTime File Format (checked 11 October 2026).