1- Molar relationship: The mesiobuccal cusp of the upper first molar occludes with the groove between the mesiobuccal and middle buccal cusp of the lower first molar.The distobuccal cusp of the upper first molar contact the mesiobuccal cusp of the lower second molar

2- Crown angulation: All tooth crowns are angulated mesially (mesiodistall tip).

3- Crown inclination: Indication refers to the labiolingual or buccolingual inclination of the crowns of the teeth.

A. Incisors are inclined toward the buccal or labial surface.

B. Upper posterior teeth are inclined lingually similarly from the canine to the premolars. Upper molar crowns are inclined slightly more than the canines and premolars.

C. Lower posterior teeth are inclined lingually , progressively more from canine to molars.

4- Rotations: Rotations are not present.

5- Spaces: Spaces are not present between teeth.

6- Occlusal plane: The plane is either flat or slightly curved.

From Andrews LF: Am J Orthod 62:296-309, 1972.
Figure

Angle’s class I molar relationship is that the mesiobuccal cusp of the upper first molar occludes in the mesiobuccal groove of the lower first molar

Andrew’s class I molar relationship is that the distal cusp of the upper first molar occludes with the mesial marginal ridge of the lower second molar