This is my spanking ruler. It's not a real measuring device, instead it was specifically designed to smack a bottom. It's a little thicker than a regular ruler and is just about a foot long. It has been coated with several layers of sealant. That's important for real use because a typical ruler can be very prone to splintering; and no one wants that. The edges have been smothed and coated plus the extra thickness gives it strength.
It gives a good bite because it concentrates the sting to a narrow band. I'd like one that is longer so I could reach both cheeks at once, but this is nice for lighter use. It is very effective on the undercurve of the bottom and the crease between the cheeks and upper thighs. I can aim very precisely with this implement. Excellent for a school-type scene and it makes a nice crack when I smack it into my palm as a lead in to the spanking.