The diameter of cigarettes and filter rods has a direct effect on the pressure drop and the ventilation and therefore on the smoking performance or user experience of the assembled product. Furthermore differences in the diameter of tobacco rod, and all internal segments more generally, might cause assembly problems. The diameter of the final product will also influence the packing. The diameter is a principal measure for the quality control of tobacco product manufacture. This guideline is intended as a reference for manufacturers of tobacco products and filter rods as to the best practice for the measurement of diameter, so as to consistently obtain the most accurate and most reliable measurements despite the many external influences that can affect the measured value.