In order to develop the mandible and to correct anterior crowding on young patients in the mixed dentition, Dr. William designed an expansion appliance for moving primary and permanent molars buccally. The appliance is fitted with an expansion screw soldered to the bands that are themselves bonded to the second primary molars. A nickel titanium wire rests on the lingual surface of the incisors in order to move these teeth buccally. With adjustments, the strength of this wire can be concentrated on one or more teeth. The William, comfortable to wear and easy to adjust, is patient friendly. Moreover, since the appliance is bonded, lack of patient cooperation does not overly prevent efficient treatment. Note that according to Dr. William, the central incisors, not the lateral ones, must erupt before starting treatment.