This tool is used for determining if a particular fret is higher than the other two around it on your musical instrument. It is made out of aluminum and the sides measure 1" x 2.5" x 3.5" x 4.5" x 1/16" thick.  The edges are diamond honed for uniform straightness

Fret placement on musical instruments are often done by hand and misalignment of the frets as measured by the edge is only an indicator and may not mean a defect in the quality or workmanship of the instrument(s) being checked.