To better assess the efficacy of mechanical preparation of root canals, transparent specimens of 510 extracted maxillary central incisors were investigated for thickness and curvature of the root canal, condition of any accessory canals, and location of the apical foramen. Over 60% of the specimens showed accessory canals that are impossible to clean mechanically. Most lateral branches were small, 80% were the size of a #10 reamer or less, and only 3% were thicker than a #40 reamer. Apical foramina located away from the apex were observed in 45% of the teeth, although nearly 80% of all foramina were within approximately 0.5 mm of the apex, and 95% were within approximately 1.0 mm. Data on the thickness and curvature of the main canal showed that normally it is adequately prepared when reached with a #60 reamer to the apical constriction and supplemented by flare preparation.