Tiger Woods has a practice drill he calls the 9 windows. The goal is simple to describe and demanding to execute: hit nine different types of shots, one after another, each intentionally different from the last. A high draw, then a high straight, then a high fade. Drop to medium height and repeat. Three rows, three columns. Nine distinct ball flights, produced by systematically varying exactly two things: where your clubface points at impact, and what direction your club is swinging through the ball.
Most golfers who slice the ball do not understand why they slice it. They think about swinging left, or keeping their elbow tucked, or rotating their forearms. What they rarely think about, because nobody told them, is the precise mathematical relationship between face angle and swing path that determines every shot shape. Once you understand it, golf’s ball flight stops looking like an unpredictable mystery and starts looking like a system with exactly two inputs.
This is that system.
The D-Plane
When a golf club strikes a ball, two things determine where the ball goes: face angle and swing path (also called club path).
Face angle is where the clubface is pointing at the moment of impact, measured relative to the target line. Aim the face left, and the face is closed. Aim it right, it’s open. Square means pointing directly at the target.
Swing path is the direction the club head is traveling through the impact zone, also measured relative to the target line. A path moving right of the target (for a right-handed golfer) is in-to-out. A path moving left is out-to-in.
The old instruction model said the ball starts in the direction of the swing path and curves based on the spin imparted by the face. TrackMan launch monitor data showed this was wrong. The ball actually starts predominantly in the direction the face is pointing: roughly 75–80% of its launch direction is determined by face angle, not path. The path matters, but primarily for curve.
The physics behind this is called D-Plane theory, introduced by physicist Theodore Jorgensen and popularized by coaches using launch monitor technology. The ball starts in the direction between the face and path, weighted heavily toward the face, and then curves away from the path. A face open to the path imparts sidespin that curves the ball right (for right-handers). A face closed to the path curves it left.
The key relationship: face-to-path difference determines curve. A face 5° open to the path will fade. A face 5° closed to the path will draw. The size of the difference determines how much the ball curves.
The 9 Windows
Three face positions × three swing paths = nine distinct outcomes. Tiger’s drill is a structured tour through all of them.
| In-to-Out Path | Straight Path | Out-to-In Path | |
|---|---|---|---|
| Closed Face | Hook | Pull Hook | Pull |
| Square Face | Draw | Straight | Fade |
| Open Face | Push | Push Slice | Slice |
The face position describes where the face points relative to the target at impact. The path describes the direction the club is traveling. Together they determine both where the ball starts and how much it curves.
Explore each window below:
Reading the visualization: The ghost trail shows the full trajectory. The D-Plane diagram (bottom-left) shows face and path directions from a top-down view; the shaded area between them is the D-plane, and the ball launches into it. The club face view (top-right) shows how far the face is open or closed relative to the square reference line. Use the minimap or ← → keys to switch between all nine shots.
Developing Face Awareness
Understanding the 9 windows intellectually is the easy part. The hard part is developing the physical awareness to know what your clubface is doing at impact, and being able to change it on demand.
Most golfers have no clear sense of where their face is pointing when it contacts the ball. The swing happens in under half a second. The wrists rotate, the forearms cross, the grip pressure shifts, and at the end of it, something about where the face ends up is determined by dozens of micro-adjustments that happen below conscious awareness. The 9 windows drill is a tool for making those adjustments conscious.
Tiger’s method is deliberate: pick a specific window before you swing, not after. Decide you’re hitting a high draw. Then make decisions (grip, setup, swing thought) that should produce that draw. Hit the ball. Observe where it went and how it curved. Adjust. Repeat. The drill is not about hitting nine different shots; it’s about building a feedback loop between intention and result that tightens over thousands of repetitions.
The most useful place to start is face angle, because it has the largest effect on starting direction. Two drills sharpen it quickly:
Slow-motion rehearsals at impact position. Stop the club at the moment it would contact the ball and look at the face. Is it square to the target line, or rotated? Most golfers are surprised by what they find. Do this in front of a mirror or on video. Build a physical memory of what a square face feels like from the inside.
Exaggerate first, then refine. Tiger’s drill works by exploring the extremes. Hit intentional snap hooks (face way closed, path way in-to-out). Then hit intentional push-fades (face way open, path way out-to-in). Make the exaggerated versions vivid and repeatable before narrowing toward the middle cells. When you can produce a banana slice and a hard hook on demand, you have far more control over the spectrum between them.
The goal is not to eliminate all shot shapes. It’s to narrow the range of unintentional ones. A golfer who can only hit a straight ball is fragile under pressure; a golfer who understands their pattern and can deliberately shift it in either direction has real command. That’s what the drill builds.
Appendix A: The Banana Slice and the Pull Hook
The outer corners of the 9-window grid (Hook and Slice) produce the most exaggerated ball flights because both conditions amplify each other.
A slice (bottom-right cell): the face is wide open and the path is out-to-in. The ball starts right because the face points right. Then it curves further right because the face is even more open relative to the already-rightward path. Beginners trying to fix a slice often swing harder out-to-in to correct a perceived leftward miss, which makes the path worse and amplifies the slice.
A banana slice is the same scenario made extreme: a face 15–20° open, a path 10–15° out-to-in. The ball starts well right of target and curves dramatically further right, sometimes beyond 90° of total deviation.
A hook (top-left cell): face very closed, path very in-to-out. The ball starts left and curves further left. Beginners who over-correct a slice by closing the face, without fixing the path, often create this.
The fix for both: narrow the face-to-path angle. You can adjust the face, the path, or both.
Appendix B: Gear Effect
There is a third factor that modifies ball flight from any given face and path combination: where on the face you actually strike the ball.
A modern driver face is curved both horizontally (bulge) and vertically (roll). When you strike the ball off-center, the face twists slightly at impact. On a toe strike, the face twists open momentarily, imparting extra draw spin. A heel strike twists the face closed, imparting extra fade spin.
This is gear effect, named by analogy to meshing gears. The ball and face behave like two gears at the contact point: the face moving one way causes the ball to spin the other.
The practical implications:
- A toe strike curves left more than face/path numbers alone predict
- A heel strike curves right more
- The bulge on the face is intentionally designed to compensate: the face naturally aims further right for toe hits (to pre-correct for the expected leftward gear effect) and further left for heel hits
Low-handicap players who fade the ball often tee it slightly toward the heel side, using gear effect to add draw spin that counteracts their natural fade, without changing their swing.
Understanding face, path, and gear effect together gives you the full picture. Face and path set the baseline trajectory. Gear effect is the correction term. Tiger’s 9 windows drill is about mastering face and path. Gear effect, when it enters, is a refinement layered on top of an already well-controlled shot.