The importance of the horizontal dimensions or working width (WW) of the root canal system has not been addressed in most of the researches. In long oval or flat canal preparations, the WW is very important as it alters the operator to the possibilities of incomplete root canal preparation. The concepts, techniques, and technologies to measure initial horizontal dimensions and to determine final horizontal dimensions accurately or properly are underdeveloped. As working length , working width in endodontics is often neglected but is very important for successful shaping and cleaning of root canal systems. This book highlights this importance.