Hello, Bill. I believe the ruler is specific to the hat trade, although it would be handy for inside measurements from 5" to 9". The wood body is exactly 5" long, and the graduations on the wood read from the open brass end. The brass slide reads from 5" to 9". With the slide extended ,the graduations on the slide include the length of the wood body. All of this seems to co-incide with the scales on the back, which start at 5-7/8 and end at 8. In the photo the slide is exactly 2" from the body, so it is reading 7". Regards, Lou