Words are failing me again.
It's a tool. We used to use it in high school when we'd take a grease pencil and mark on a print our cropping instructions, and then the printer would crop and insert the photo into the newspaper/yearbook.
It's like two L-shaped rulers connected together, so that you could crop the photo to the size of the space, but you had leeway within the print to crop tighter or looser ....
I want one. Do they still sell them? And where?