Coefficient of restitution (COR) measures how much energy a clubface returns to the ball on impact, on a scale from 0 (no rebound) to 1 (a perfectly elastic bounce). In practice it's a measure of how much a thin driver face flexes and springs back at the moment of contact, and the USGA and R&A cap conforming drivers at a COR of 0.830 (set in 1998, with test tolerance built in), and virtually every modern driver on the market is built right up against that ceiling. A higher COR raises smash factor — the ratio of ball speed to clubhead speed — which is why a 0.830 face lets a well-struck shot approach the practical 1.50 smash factor ceiling. Since the early 2000s, governing bodies test the related 'characteristic time' rather than COR directly, but COR remains the physical property the rule was written to control.

Coefficient of restitution (COR) measures how much energy a clubface returns to the ball on impact, on a scale from 0 (no rebound) to 1 (a perfectly elastic bounce). In practice it's a measure of how much a thin driver face flexes and springs back at the moment of contact, and the USGA and R&A cap conforming drivers at a COR of 0.830 (set in 1998, with test tolerance built in), and virtually every modern driver on the market is built right up against that ceiling. A higher COR raises smash factor — the ratio of ball speed to clubhead speed — which is why a 0.830 face lets a well-struck shot approach the practical 1.50 smash factor ceiling. Since the early 2000s, governing bodies test the related 'characteristic time' rather than COR directly, but COR remains the physical property the rule was written to control.