Deviations in other spatial axes, asymmetric deviations, functional faults and other therapy-related features are not recognised. Another shortcoming is the lack of a theoretical basis of this purely descriptive classification system. Among the much discussed weaknesses of the system is the fact that it only considers the static occlusion, that it does not take into account the development and causes (aetiology) of occlusion problems and it disregards the proportions (or relationships in general) of teeth and face. Thus, numerous attempts have been made to modify the Angle system or to replace it completely with a more efficient one, but Angle's classification continues to prevail mainly because of its simplicity and clarity. Well-known modifications to Angle's classification date back to Martin Dewey (1915) and Benno Lischer (1912, 1933). Alternative systems have been suggested by, among others, Simon (1930, the first three-dimensional classification system), Jacob A. Salzmann (1950, with a classification system based on skeletal structures) and James L. Ackerman and William R. Proffit (1969). Besides the molar relationship, the British Standards Institute Classification also classifies malocclusion into incisor relationship and canine relationship. Class I: The lower incisor edges occlude with or lie immediately below the cingulum plateau of the upper central incisors  Class II: The lower incisor edges lie posterior to the cingulum plateau of the upper incisors  Division 1 – the upper central incisors are proclined or of average inclination and there is an increase in overjet  Division 2 – The upper central incisors are retroclined. The overjet is usually minimal or may be increased. Class III: The lower incisor edges lie anterior to the cingulum plateau of the upper incisors. The overjet is reduced or reversed. Class I: Mesial slope of upper canine coincides with distal slope of lower canine  Class II: Mesial slope of upper canine is ahead of distal slope of lower canine  Class III: Mesial slope of upper canine is behind to distal slope of lower canine  Crowding is defined by the amount of space that would be required for the teeth to be in correct alignment. It is obtained in two ways. 1) by measuring the amount of space required and reducing this from calculating the space available via the width of the teeth. Or 2) by measuring the degree of overlap of the teeth. The following criteria is used:  0-4mm = Mild crowding  4-8mm = Moderate crowding  >8mm = Severe crowding  Extra teeth, lost teeth, impacted teeth, or abnormally shaped teeth have been cited as causes of crowding.