[LargeFormat] Measuring square?

[rstein]

Dear Friends,

     I have been trying to remember what it is called, but it escapes me. I
mean the measuring device that looks like two L-shaped rulers joined over
each other with a locking collar that enables you to set an aspect ratio and
then open and close the rectangle as you frame up a picture.

     I want to buy one to start drawing cropping lines on contact sheets -
does anyone know what they are called and where they are sold?

     Uncle Dick