The overjet is reduced or reversed. Canine relationship by Ricketts
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 of teeth
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 criterion is used:0-4mm = Mild crowding
4-8mm = Moderate crowding
>8mm = Severe crowding

Causes
Genetic (inheritance) factors, extra teeth, lost teeth, impacted teeth, or abnormally shaped teeth have been cited as causes of crowding. Ill-fitting dental fillings, crowns, appliances, retainers, or braces as well as misalignment of jaw fractures after a severe injury are also known to cause crowding. Tumors of the mouth and jaw, thumb sucking, tongue thrusting, pacifier use beyond age three, and prolonged use of a bottle have also been identified.Lack of masticatory stress during development can cause tooth overcrowding. Children who chewed a hard resinous gum for two hours a day, showed increased facial growth. Experiments in animals have shown similar results.
Bolton developed a method to calculate the ratio between the mesiodistal width of maxillary and mandibular teeth and stated that a correct and harmonious occlusion is possible only with adequate proportionality of tooth sizes. Boltons formula concludes that if in the anterior portion the ratio is less than 77.2% the lower teeth are too narrow, the upper teeth are too wide or there is a combination of both. If the ratio is higher than 77.2% either the lower teeth are too wide, the upper teeth are too narrow or there is a combination of both. Other conditions
Other kinds of malocclusions can be due to or horizontal, vertical, or transverse skeletal discrepancies, including skeletal asymmetries. Increased vertical growth causes a long facial profile and commonly leads to an open bite malocclusion, while decreased vertical facial growth causes a short facial profile and is commonly associated with a deep bite malocclusion. However, there are many other more common causes for open bites (such as tongue thrusting and thumb sucking) and likewise for deep bites.The upper or lower jaw can be overgrown (macrognathia) or undergrown (micrognathia). It has been reported that patients with micrognathia are also affected by retrognathia (abnormal posterior positioning of the mandible or maxilla relative to the facial structure). These patients are majorly predisposed to a class II malocclusion. Mandibular macrognathia results in prognathism and predisposes patients to a class III malocclusion.Most malocclusion studies to date have focused on Class III malocclusions. Genetic studies for Class II and Class I malocclusion are more rare.