Class II Amalgam Preparation / page 6

RETENTION FORM Proximal displacement is prevented by occlusal dovetails.  Convergent buccal and lingual walls in the isthmus and proximal box prevent occlusal displacement of the restoration.

RETENTION FORM - Grooves The proximal box is retained by retention grooves placed buccally and lingually at their respective  axiobuccal and axiolingual line angles.  Grooves follow the convexity of the axial wall, and do not penetrate the axial wall.

They are 0.25 mm in depth at the axiobuccogingival and axiolinguogingival point angles and fade to zero depth as they approach the axiopulpal line angle. Retention grooves are prepared entirely in dentin to prevent creating any unsupported enamel.