What it measures
Face to Path compares two numbers measured at impact: where the face points and where the club is traveling. It is calculated as Face Angle minus Club Path, in degrees. It ignores the target completely. A face 3 degrees right on a path 5 degrees right gives minus 2, and so does a face 7 degrees left on a path 5 degrees left.
For a right-handed golfer, a positive value means the face is open to the path, which tilts the spin axis right and curves the ball right. A negative value means it is closed to the path and the ball curves left. Zero means face and path match, and a centered strike will fly without curve.
Typical numbers
Small numbers create big curves. Some examples for centered strikes: a tour driver carrying 275 yards with a Face to Path of minus 2 curves about 19 yards left, and plus 5 curves about 44 yards right. A tour 6 iron carrying 183 yards curves about 8 yards with plus 2 and about 20 yards with minus 5. As a rough guide, each degree is worth about 12 yards of curve on a 300 yard drive.
Skilled players usually keep stock shots within about 1 to 3 degrees of zero, on the side that produces their preferred shape. Everyday golfers often show 4 degrees or more, and slices commonly come with a clearly positive number. Curve per degree is larger with the driver than with short irons, because lower loft lets the spin axis tilt more.
What changes it
Anything that changes face or path changes this number. Opening the face or swinging more to the left raises it; closing the face or swinging more to the right lowers it. Players often compensate without noticing: an out-to-in path tends to invite a closed face to keep the ball from going right, which can turn a slice into a pull.
What it changes
Face to Path is the main input for Spin Axis and therefore curve. With a centered strike the ball curves toward where the face points and away from the path. How much it curves also depends on spin loft and speed, so the same number bends a driver far more than a wedge.
Off-center strikes, mainly with drivers and woods, can override it through gear effect: a toe strike can turn an open Face to Path into a draw. Comparing Face to Path with Spin Axis or D Plane Tilt shows when that is happening.
How to use it
Use Face to Path to explain curve and Face Angle to explain start line. If the curve is too big, the fix is usually bringing face and path closer together rather than changing only one of them.
Bay drill: with a 7 iron, try to hit three shots with Face to Path between 0 and plus 2 and three between 0 and minus 2. Write down what you changed to switch. Then repeat with a driver and notice how much more the same numbers bend the ball.