The 90 degree rule is an important skill to predict where the cue ball will be after executing basic and complex shots after hitting the object ball.
It is when the cue ball hits a target ball without any english and the two balls always leave at 90 degrees.
In other words, the path between the cue ball and the object ball after impact will be 90 degrees.
Now let’s hit the 1-ball into the corner pocket, draw a line through the 1-ball, the cue ball slides over hitting the 1-ball without english and see how it makes a 90 degree angle.
Again, we are playing by the 2 ball.
The first thing we do is visualize a line that passes through the 2-ball and ends in the pocket, once we visualize this line, we can now see the trajectory of the 90 degree angle that the cue ball will make after contacting the 2-ball.
Another way to visualize how the balls will shoot out after the shot, with your hand make an L shape between your index finger and thumb.
The index finger should be straight on the line of impact of the object ball and the thumb indicates the direction where the cue ball will make its trajectory is called the tangent line which results in the 90 degrees.
The distance the cue ball travels along the tangent line after impact increases with the speed of the cue ball and the angle of cut.
There is an exception to this rule when the cue ball hits the object ball perfectly on center without any angle of cut.
In this case the cue ball will be completely stopped or a stop shot.
The 90 degree rule is widely used to prevent you from pocketing the cue ball, to separate balls that are close together, to execute carom shots and to avoid obstacles.
For example, if you are going to hit a target ball and you see that with the 90 degree rule you are going to hit the cue ball as well, what you must do is use the running or backspin effects to avoid losing the shot.
It is important that you also learn the 30 degree rule.