Summary. This study reports the effect of the step‐bad enlargement technique of root canal preparation on the shape of the curved root canal of extracted human teeth. This was achieved by superimposing tracings made from photographic prints obtained by radiographs of teeth before canal preparation, after apical preparation and after coronal flaring. After apical preparation, the root canal sometimes demonstrated an hour‐glass shape. Flaring the coronal portion of the canal was often successful in opening out the narrowest portion, the elbow, of this hour‐glass shape.