Abstract: An endodontic device for instrumenting a root canal includes an elongated wire, a handle at one end of the wire, and one or more movable stops extending a portion of the wire and abutting on the handle. The length of the portion of the wire which protrudes from the stop(s) constitutes the working length for the root canal treatment. The working length is maintained the same during the treatment, since the stopping means is sandwiched by two stiff objects, namely, the tooth being treated and the rigid object. Also disclosed are (1) a set of movable stops of specific thicknesses for precisely setting a range of working lengths, and (2) a container for storing the set.