This is made in Sweden by Truemiller. We asked them to modify their original design to the US measurement system. When we did that, we also asked for a color change so that it was easy to see the difference. This is a great tool to find your distance on known/estimated target sizes.
The outer ring represents your target size in Mils this should be a known size. The inner ring is the size the target takes up in your scope. If you have a second focal plain scope, keep in mind the power setting that your mils are accurate. You must be super accurate on this. I found on my scope, I get a more accurate reading if I move into an area that is marked with 10ths, instead of half mils.
The yardage is displayed in the window opening. The opening also give reference to four different angles of measure that the target may be located up or downhill from you.