In contemporary orthodontic practice, a large number of Class I and Class II patients, with and without crowding, are treated without premolar extractions. A wide range of philosophies and devices have been introduced with the aims of providing more space within the arches for the accommodation of the teeth, and influencing the relationships between the upper and lower arches. Some of these philosophies and devices are new--others have been recycled from previously-used models, which may or may not have been discarded for a number of reasons. Much descriptive and anecdotal material has been presented to support these developments. There have also been isolated studies of the effects of various mechanical component parts on dentofacial structures, some using homogenous subject-material, others using quite non-homogenous material. Little quantitative assessment has been made, however, of the effects of holding the E-spaces, uprighting the permanent first molars and redirecting the erupting permanent second molars, on attempts to provide sufficient space within the arches for alignment and levelling, correcting Class II anterior and posterior relationships and finding space for the uncomplicated eruption of the third molars. In addition, none of these space-holding or gaining effects has really been assessed alongside the effects of various attempts to orthopaedically-modify maxillary growth, if necessary, as the mandible naturally moves forward with its dentition. If a likely range of growth and treatment effects could be demonstrated in large relatively homogenous samples, one would have gone someway towards explaining the anecdotal phrase "unlocking the malocclusion", which has so often appeared in the literature. One might also have challenged another widely-held concept--that, when relieving crowding within the dental arches, one is either an extractionist or an expansionist. It might then be possible to show that, in a growing patient with a Class I or II malocclusion, crowding may well be relieved by providing space within the arches (perhaps, at the expense of the third molars), and by allowing the mandible and its dentition to move forward with growth, while forward movement of the maxilla and its dentition is inhibited.