Aspect ratio & letterboxing
The shape of your frame, plus the black bars that fill the gap when the footage shape does not match the player it lands in.
See it
What it is
Aspect ratio is the shape of your frame written as width to height: 16:9 for YouTube, 9:16 for Reels and Shorts, 1:1 for square feeds, 4:5 for the tall Instagram post, 2.39:1 for anamorphic 'scope'. Letterboxing is what happens when a wide frame is dropped into a taller container: black bars pad the top and bottom instead of cropping the picture. Pillarboxing is the same move rotated, with bars down the left and right when vertical footage sits in a wide player.
Reach for deliberate letterboxing when you want the cinema signal: matting a 16:9 timeline down to 2.39:1 instantly reads as 'film' even though you shot on a phone. Reach for real letterboxing when the alternative is cropping the subject out of frame, and reach for automatic reframing instead when you want the crop to follow the action.
Gotcha: burned-in bars are pixels, not metadata, and they never go away. Bake 2.39:1 bars into a 16:9 export, hand it to a vertical player, and the viewer gets bars on all four sides (windowboxing) with your subject shrunk to a stamp. Bake the bars only in the final per-platform export, keep an unmatted master, and remember that feed players crop the extremes of any ratio they were not given.
Ask AI for it
Place this 16:9 footage inside a 1080x1920 (9:16) vertical canvas: scale it to the full canvas width, center it vertically, and fill the empty space above and below with pure black letterbox bars. Then matte the video itself to a 2.39:1 anamorphic crop with a second pair of hard-edged black bars, no feathering, no drop shadow. Keep all titles inside the visible picture area, at least 10 percent in from every edge, so nothing sits under the bars.