How it works
A cue ball that slides on the cloth for a while starts to roll. That is natural roll: it turns forward exactly as fast as it moves. Most normal shots arrive with natural roll.
At contact, a rolling cue ball first goes along the tangent line, like a stun shot. But its forward spin still grips the cloth. It pulls the cue ball forward and bends its path. After the bend, the cue ball travels in a straight line again.
The 30° rule says where that line points. For cuts from about ¼-ball to ¾-ball, a rolling cue ball ends up about 30° away from its aim line. Near a half-ball hit it is a little more (about 34°). Near a ¼-ball or ¾-ball hit it is a little less (about 27°).
Speed changes where the bend happens, not the final direction. At low speed, the cue ball bends almost at once. At higher speed, it travels further along the tangent line before it bends.
Try it: where does the cue ball go?
Cut angle
30°
About half-ball hit
Cue ball, seen from behind
½ tip-width above 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 between ¼-ball and ¾-ball and find the ghost ball (Level 1, Lesson 5).
- 2
Picture the aim line: from the cue ball, through the centre of the ghost ball, and on beyond it.
- 3
Make a V with your index and middle finger, about 30° wide. Put the point of the V at the centre of the ghost ball, with your index finger along the aim line. Your middle finger goes on the other side of the aim line from the object ball's path, between the aim line and the tangent line. Turn your hand over if you need to. Your middle finger now points where the cue ball will go.
- 4
Check where that line meets a cushion, and whether a pocket or another ball is in the way.
- 5
Play the shot with a rolling cue ball: hit it in the centre or a little above, at a medium-soft speed (Level 1, Lesson 7).
- 6
Watch the cue ball. It should leave along the tangent line, bend forward, then run out parallel to your 30° line, a little to the tangent-line side of it. The softer the shot, the closer it stays to the 30° line.
Common mistakes
Using the 30° rule on a stun shot
A cue ball with no spin at contact does not bend. It stays on the tangent line (Level 2, Lesson 1: Stun and the 90° rule).
Hitting hard and expecting the same spot
The final line points the same way, but it starts further along the tangent line. The cue ball lands further up the cushion.
Very full or very thin cuts
Outside ¼-ball to ¾-ball, the cue ball turns less than 30°. On a very full cut it carries on almost straight. On a very thin cut it hardly changes direction, so it stays close to both the aim line and the tangent line.
Drawing the V from the cue ball
The V starts at the ghost ball, where the cue ball is at contact.
Practice this
Exercise · Level 2
30° check
Set up a half-ball cut into a corner pocket, with the object ball on the diagonal, one diamond out from the pocket. Place a coin where your 30° line meets the cushion, then move it a few centimetres to the tangent-line side. Check that the path does not run into a pocket. Play 10 shots with a rolling cue ball at medium-soft speed and see how close the cue ball gets.
Exercise · Level 2
Stun or roll
Play the same half-ball cut twice, once as a stun shot and once with a rolling cue ball. Mark where the cue ball hits the cushion each time. The gap between the two marks shows how much the roll bends the path.
Check yourself: 10 half-ball cuts with a rolling cue ball. 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: check your V once at home. Lay a protractor or a printed 30° angle on the table and spread your fingers to match it, so you know how wide your 30° is.