Methods and apparatus providing a critical set of endodontic instruments. The set includes a first endodontic instrument, a second endodontic instrument, and a third endodontic instrument. Each endodontic instrument includes a working portion that has a cone-like shape. Each working portion having an effective contact area defined by the exposed surface area of the working portion's respective cone-like shape. The difference between the effective contact areas of the first endodontic instrument and second endodontic instrument being substantially the same as the difference between the effective contact areas of the second endodontic instrument and third endodontic instrument.