"Cropping guide"
 

Below is a tool that I use often. Most good photo processors and camera stores keep this tool handy.

There are very few instances where the photo image I shot ends up being the exact composition I paint. Laying this guide over my standard 4x6 print, I can crop the image using the 5x7 ratio, or the 8x10 ratio side. I would not know how to function with out this tool.

It can also be used to help compose any scene in life. Granted, what you see through the rectangle will be a fixed ratio (height to width) as mentioned above but it can help to view the scene.

I purchased mine at my camera store for $8.50.

I have one of therse too, and is very helpful when you show clients the idea of what their painting's compostion will look like.