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Most shots in pocket billiards are angle shots or "cuts," meaning the cue ball does not strike the object ball dead on. The "thinner" (more glancing) the collision is, the greater the angle the object ball will travel at, relative to the trajectory of the cue ball. An excellent way to estimate this effect is to sight along the planned trajectory of the ball. At the moment of collision, how much will the cue ball "overlap" the object ball from your perspective? The answer tells you how "full" the collision is: A dead-on shots overlaps completely. You could say it has a "fullness" of 1. If the cue ball covers of the object ball, the hit is full. The graph of these two quantities is not quite linear, but it's close enough that you can estimate by adding 15 every time you subtract fullness. Alternatively, use these more accurate measurements: A direct hit (fullness 1) results in a cut angle of 0. The object ball continues along the same path as the cue ball. A shot sends the object ball out at 14.5. A shot sends the object ball out at 30. A shot sends the object ball out at 48.6. Past fullness, it becomes difficult even to estimate how much of the ball is covered. More importantly, the cut angle rises more and more steeply, so tiny errors can have large effects. These glancing shots require plenty of practice and good technique even once you've figured out where to aim. If you can, look for another shot you can take. If the description of fullness doesn't help you, try the "ghost ball" approach: Imagine a straight line segment from the pocket to the center of the object ball. Extend this line slightly past the object ball. Imagine a "ghost ball" at this spot, squarely on this line and touching the object ball. To hit the object ball into the pocket, you should aim at the center of the "ghost ball." A kiss shot involves caroming the cue ball off ball A so it can strike ball B. If you're playing a game that allows kiss shots, remember this rule: if ball A is touching a rail, the desired cut angle is of the angle formed by the three balls. For example, if the angle with ball A as the vertex is about 45, the cut angle you want to achieve is about 15. The fullness rule above tells us that a full collision should produce this angle.
Learn the basics. Estimate the fullness of the hit. Predict the angle based on the fullness. Use caution for very thin shots. Aim with the ghost ball method instead. Follow the thirds rule for kiss shots.