hello, i have a question regarding finish line design (chamfer and shoulder) for PFM crown preparation. i understand that for PFM preparation, the lingual should be chamfer because metal sits on it and buccal should be shoulder because porcelain sits on it. i understand that porcelain is likely to fracture if it sits on chamfer design, but why? also, i am very confused as to how to go about the transition area going from chamfer to shoulder in the proximal margin area. do i sort of blend in the two designs into a transition zone or do i make a uniform chamfer design on the lingual half and abruptly began making a shoulder design on the buccal half?