Stylus roughness instruments record the up and down distances of an external stylus traversing across a surface. They count the number of peaks (Pc) encountered along a straight 12.5 mm (0.5 in) line. The PosiTector RTR-P counts the number of peaks from a replica tape surface area that is approximately 16 mm2 (0.63 in2). Although stylus roughness instruments yield a 2D measurement in peaks/mm (peaks/in) and the PosiTector RTR-P series yields measurements in peaks/mm2 (peaks/in2), the two methods correlate well.