Our occlusal relationship is classified into three different types (Class I, Class II, Class III) based upon the relationship of upper and lower first molars.  Class I occlusion/malocclusion is the occlusion when upper first molar buccal cusp bites (occludes) on lower first molar buccal groove.  Class II malocclusion is when upper first molar bites ahead (mesially) of lower first molar; Class III malocclusion is when upper first molar bites behind (distally) of lower first molar.

Classification of malocclusion is important when making the diagnosis and treatment plan for the bite problem.