Calipers designed to slide over a ruler.
I printed a set of calipers (http://www.thingiverse.com/thing:48413) but found the markings too hard to read, so I adapted the design to fit over a metal ruler I already have.
The ruler is 26.5mm wide and 1mm high, so the design fits those measurements. I've included the sketchup files if you want to modify them.
The top fits over 1cm of the ruler, so you need to subtract 1cm from the measurement.