What is the ideal total occlusal convergence (taper) angle for a full crown preparation?

According to Shillingburg's Fundamentals of Fixed Prosthodontics, the recommended total occlusal convergence (TOC) for full crown preparations is 10–20°. The theoretical ideal is approximately 6° (3° per axial wall), which maximizes retention, but this is difficult to achieve clinically. Studies show that clinically acceptable preparations typically fall within 10–20° of total convergence. Preparations with more than 20–25° of convergence have reduced retention. All-ceramic crowns relying on adhesive cementation may tolerate slightly higher convergence due to the additional retention from resin bonding.