All preparations should have the maximum height and minimum taper for optimal resistance and retention from consistent with chemical situation. To achieve this and to permit an adequate thickness of restorative material without over-contour, the surface of the preparation should mimic that of the intended restoration, both occlusally and axially.

UNDERCUT-FREE PREPARATION - there must be one point above the preparation from which all the margins and internal line angles can be seen.

A single PATH OF INSERTION over as great a distance as possible - this is achieved by preapring opposing walls to be near-parallel to give maximum retention. The position of the adjacent teeth should be considered as they may overhang the margins of the prepared tooth. The path of insertion is therefore dictated by the adjacent teeth. 

The OPPOSING WALLS in the gingival half of the preparation should be made near-parallel. The occlusal third to half will usually be more tapered as a result of the two plains of labial reduction required to provide sufficient room for the restorative material within the original tooth contours.

With SHORT CLINICAL CROWNS there is an increased risk of failure because of the short insertion path. Preparation length can be increased by crown lengthening, and resistance form may be improved by the use of grooves, slots or boxes and by converting sloping surfaces itno vertical and horizontal components.

OCCLUSAL REDUCTION should follow cuspal outline to maximize retention and minimze tooth reduction. For porcelain fused to metal crowns and for gold crowns these distances are 2 and 1 mm, respectively.

The FINISHED MARGIN position and type are determined by the gingival contour, the nature of the restorative material, the presence or absence of a core magin and the choice of luting agent. Whenever possible, the margin should be supragingival following the natural gingival contours. Finish margins should ideally extend at least 1mm past core margins to rest on sound tooth tissue.