1. Headgear, a dilemma: which, when, and why;Hilgers;Trans Eur Orthod Soc,1971
2. Directional forces;Merrifield;Am J Orthod,1970
3. Controlling the magnitude, direction, and duration of extraoral force;Armstrong;Am J Orthod,1971
4. Directional edgewise orthodontic approach;Hickman;J Clin Orthod,1974
5. Directional forces;Berman;Br J Orthod,1976