Letterboxing & pillarboxing
Also known as: letterbox, pillarbox, black bars on video, aspect ratio bars
Letterboxing adds black bars above and below a video, and pillarboxing adds them to the left and right, so footage of one aspect ratio fits a screen of another without stretching or cropping. The bars preserve the original framing; they are not part of the picture.
- Letterbox = top/bottom bars; pillarbox = side bars
- Bars preserve the original aspect ratio
- Can be baked into the file or added at playback
Why the black bars appear
Letterboxing appears when wide footage — a cinematic 21:9 clip — plays on a less-wide screen like 16:9: bars fill the top and bottom so the full width still fits. Pillarboxing is the reverse, with bars on the sides when a narrower or vertical clip plays on a wider screen.
Both exist to respect the original aspect ratio. The alternative would be stretching the image, which distorts it, or cropping, which throws away part of the frame. The bars simply pad the difference.
Editing and file considerations
If bars are baked into the file, they are encoded as real black pixels and add to the frame; cropping them out in an editor or compressor produces a tighter video at the native aspect ratio. Often, though, players add the bars at playback and the stored file has none.
Black bars compress very efficiently, so baked-in letterboxing adds little to file size compared with the picture itself, which is driven by resolution and bitrate.