The edit is locked. The colour is graded. The music is licensed. Sam sends the review link at 10:02 and gets a reply at 10:09: "Looks great, but why are there black bars on the sides?"
On Sam's monitor there are no bars. On the client's laptop there are. Nobody has done anything wrong, and yet the video looks unfinished.
What the bars are telling you
Black bars are not a defect in the file. They are a player's polite way of saying "your picture and my window are different shapes". It shows the whole picture and fills the leftover space with black.
- Bars on the left and right (pillarboxing) mean the video is narrower than the player.
- Bars on the top and bottom (letterboxing) mean the video is wider than the player.
The client sees side bars, so the video is narrower than their player. Sam opens the export settings.
Finding the mismatch
The export is 1440 × 1080. Divided out, that is 1.333: a 4:3 frame. The timeline had been created from the first clip dropped into it, an old interview recorded in 4:3. Everything else was shot in 16:9 and had been quietly scaled down to fit inside the 4:3 timeline.
On Sam's editing monitor the viewer filled its panel, so it looked fine. On a 16:9 laptop, the 4:3 video was pillarboxed. Worse, the 16:9 shots inside it already had thin bars of their own, top and bottom. Bars on all four sides, also called windowboxing, is a clear sign of a shape mismatch somewhere in the pipeline.
The fix
- Change the sequence to 1920 × 1080 (16:9), the shape of the delivery and of most of the footage.
- Reframe the one 4:3 interview. Sam has two options: pillarbox it on purpose with a soft background, or crop 25% of its height to fill the frame. The interview is a tight head-and-shoulders shot, so a gentle crop works.
- Re-export and check the numbers before sending: 1920 ÷ 1080 = 1.778. It matches the player.
The habit that prevents it
Since then, Sam does one thing at the start of every project: set the sequence size by hand, before any clip goes in. They also do a ten-second check before every review link: divide width by height, and compare it with where the video will play.
If the video's number is larger than the screen's, expect letterboxing. If it is smaller, expect pillarboxing. If they match, there are no bars.
The video aspect ratio guide covers letterboxing, pillarboxing and conversions in more depth. You can check any frame's shape in the calculator.