Letterboxing.

Padding an image with solid bands, usually black or white, so it fits an aspect ratio it was not composed for. Sellers do it to reuse one file across several poster sizes, and platforms do it on their side when a file misses the product: ArtStation tells artists to avoid letterboxing, while Redbubble centres and pads non-standard Art Prints with minimum borders of 1-2 inches to fit each paper size. Padding looks harmless on screen, where the bands melt into the page. On paper it reads as a mistake, a strip on two sides that the print was never composed for. Build each aspect ratio as its own file, because the customer who returns a poster with a stripe down one side is right, and the reprint is on the seller.