What Is the 90 Degree Rule in Pool? Predict Where the Cue Ball Goes

Quick answer: The 90 degree rule in pool says that when the cue ball hits an object ball with no top or bottom spin at impact, called a stun hit, the cue ball and object ball move apart at about a 90 degree angle. The object ball goes along the line between the centers of the two balls, and the cue ball goes off along a line at a right angle to it, called the tangent line. Knowing this helps you predict where the cue ball will go and avoid scratches.

The 90 degree rule is one of the most useful ideas in pool. It is the starting point for learning position play.

How the 90 Degree Rule Works

The cue ball hits the object ball on a cut.
The object ball travels along the line through the centers of both balls at contact.
The cue ball leaves along the tangent line, which is about 90 degrees from the object ball’s path.

This only works cleanly when the cue ball is sliding, with no roll, at the moment it hits the object ball. That is a stun hit. Learn more in our post on what is a stun shot in pool.

Use Your Hand to See the Line

A simple trick many players use:

Make an “L” shape or a peace sign with your fingers.
Point one finger where the object ball will go.
Your other finger shows the tangent line, where the cue ball will go.

This gives you a quick picture before you shoot.

How Spin Changes the Path

Spin at impact	Cue ball path
Stun (no spin)	Along the tangent line, about 90 degrees
Follow (top spin)	Curves forward, less than 90 degrees from the object ball’s path
Draw (back spin)	Curves back, more than 90 degrees

Rolling balls naturally have some follow when they reach the object ball. Many players learn a related idea called the 30 degree rule for rolling cue balls on some cuts.

Why Speed Matters

On a long shot, a cue ball hit with a little draw may lose its spin and arrive with stun. Hit softly and the cue ball starts to roll, adding follow. So speed and spin together decide whether you get a true 90 degree path.

Use It to Avoid Scratches

Before you shoot, check the tangent line. If it points at a pocket, you may scratch. Change your spin or speed to move the cue ball off that line. For more tips, see how to stop scratching in pool.

Use It for Position

The tangent line tells you where the cue ball starts to go. Then the rails send it on from there. Combine the 90 degree rule with how to use the diamonds on a pool table to plan your next shot. Read how to play position in pool for more.

When It Does Not Apply

Straight in shots: On a full hit with stun, the cue ball stops. There is no angle.
Very thin cuts: The cue ball keeps going almost straight. It still follows the tangent line, which is nearly the same as its original path.
Balls with spin: Follow or draw changes the angle, as shown above.

Practice Drill

Set up a cut shot into a corner pocket with an object ball about a foot from the pocket. Hit with a firm stun stroke slightly below center. Watch the cue ball leave at a right angle. Then try follow and draw and compare the paths.

The Bottom Line

What is the 90 degree rule in pool? When the cue ball hits an object ball with stun, the two balls split at about a right angle. The object ball goes along the line of centers, and the cue ball follows the tangent line. Follow makes the angle smaller, and draw makes it bigger. Use it to avoid scratches and plan position.

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