Stretcher bar.

The wooden frame a printed canvas is stretched over and fastened to at the back. Its depth decides how much image the file has to carry beyond the visible face: Prodigi adds 0.75 inch per edge for a 19 mm bar and 1.575 inch per edge for a 38 mm one, and Gelato works the sum for a 200 × 300 mm face on 2 cm bars as a 298 × 398 mm file. The same canvas size is often sold on two depths, so a file built for the slim bar and ordered on the deep one runs out of image on the sides. The lab's remaining options are the ones Prodigi lists, a flat colour or a mirror, or it stretches the image further and takes more off the front, and none of those is what the customer approved on the mockup.