ZEISS reference styli are used to determine the position and geometry of the qualification test piece, usually a sphere. The software thus recognizes the ACTUAL data of the qualification test piece, which can then be compared with the calibrated NOMINAL data in order to detect machine errors. Afterwards, all following styli installed in stylus systems can be qualified in relation to them. Master styli are defined and labeled according to stability and accuracy in a readout procedure. Optimum results can be achieved during qualification. In the M3 XXT system there are 2 different kinds of reference styli. Standard with titanium adapter and for quick set-up with a knurled plastic adapter.