Abstract:The master apical file (MAF) is the last file taken to the minor constriction (MC) of the apical foramen. Its diameter becomes the final prepared diameter of the canal at the MC. An ideal MAF preparation should provide debris removal throughout the canal, facilitate irrigation, and actually cut dentine circumferentially along the canal walls in the apical third to the level of the MC. It is essential that MAF creation does not transport the canal at any level or enlarge the MC. The final prepared canal shape should resemble a tapering funnel with narrowing cross-sectional diameters. In conclusion, the lecture will answer one question that arises more than any other: ‘How large do you instrument your cases? In addition, The lecture will also help guide clinical decisions regarding the creation of the ideal MAF size for general dentists in daily practice.