How it works
In Level 1, Lesson 6, the cue ball stopped dead on a straight-in shot. It had no spin when it hit the object ball, so all its speed went into the object ball. That is a stun hit.
On a cut shot, the same stun hit does something different. The object ball goes along the line from the ghost ball through the object ball's centre, towards the pocket. The cue ball keeps the rest of its speed and leaves at a right angle to that line. This line is the tangent line. It runs through the ghost ball, at 90° to the object ball's path.
This is the 90° rule. It works for every cut angle and every speed, as long as the cue ball has no topspin or backspin at contact. How far the cue ball travels depends on two things: the cut and the speed. The thinner the cut, the more speed the cue ball keeps. The harder you hit, the further it goes. On a nearly full cut, even a medium stroke moves the cue ball only a few centimetres. On a half-ball or thinner cut, the same stroke can send it a long way. Philip Capelle calls this a force shot, and the cue ball's path the parallel tangent line.
If the cue ball is rolling at contact, it bends forward off the tangent line. With backspin, it bends back. Lesson 2 covers the rolling cue ball.
Try it: where does the cue ball go?
Cut angle
30°
About half-ball hit
Cue ball, seen from behind
½ tip-width below centre
Tap or drag up and down. Red ring: the miscue limit.
Try a lesson shot
Show lines
What the cue ball does
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Press “Play shot”.
Drag the balls, tap a pocket to aim at it, and choose where the tip hits the cue ball. Simplified physics with typical cloth values: no side spin and no throw. The animation runs slower than real life.
Step by step
- 1
Set up a cut shot and find the ghost ball (Level 1, Lesson 5).
- 2
Picture the object ball's path: from the ghost ball, through the object ball, to the pocket.
- 3
Picture the tangent line: through the ghost ball, at 90° to that path. The cue ball will travel along it. Check where it meets a cushion, and whether a pocket or another ball is in the way.
- 4
Play a stun shot, like the stop shot in Level 1, Lesson 6. Keep the cue level. If the cue ball is close to the object ball, about 30 cm, hit it in the centre. Further away, hit it below centre: the further and softer the shot, the lower you hit. The backspin should wear off just as the cue ball reaches the object ball.
- 5
Stay down and watch the cue ball. It should leave along the tangent line, without curving.
Common mistakes
Too soft or too high
The backspin wears off too early and the cue ball rolls. It bends forward of the tangent line.
Too low or too hard on a short shot
The cue ball still has backspin at contact. It bends back of the tangent line.
Forgetting the scratch
The tangent line can point straight into a pocket, often a side pocket. Check it before you shoot. If it does, add a little follow or draw to bend the cue ball away.
Picturing the line from the object ball
The tangent line runs through the ghost ball, where the cue ball is at contact, not through the object ball.
Practice this
Exercise · Level 2
Tangent line target
Set up a half-ball cut into a corner pocket. Place the object ball on the diagonal, one diamond out from the pocket. Put a coin or a piece of chalk where the tangent line meets the cushion. Before you shoot, check that the tangent line does not run into a pocket. If it does, move the cue ball. Stun the cut 10 times and see how close the cue ball gets to the coin.
Exercise · Level 2
Three cuts, one line
Play three cuts, ¼-ball, ½-ball and ¾-ball, and put a coin on each tangent line. The cue ball always leaves at 90°, but it travels furthest on the thinnest cut.
Exercise · Level 2
Three paths
Place the cue ball about 30 cm from the object ball and play the same half-ball cut three times: in the centre, half a tip-width above centre, and half a tip-width below centre. The centre hit goes along the tangent line. The high hit bends a little forward of it, the low hit a little back. That is how good players fine-tune where the cue ball goes.
Check yourself: 10 half-ball stun cuts. In 7 of them, the object ball drops and the cue ball reaches the cushion within half a diamond of the coin. Then you’re ready for the next lesson.
Note: a quick check with your hand: lay three fingers on the cloth, centred on the tangent line. Together they are about one ball wide, so they show both edges of the cue ball's path. If any part of another ball overlaps them, the cue ball will hit it.