Pool glossary · Aiming & Physics
90-degree rule
Also called: Stun rule
What is the 90 degree rule in pool?
The 90-degree rule says that a stun (sliding) cue ball always separates from the object ball at 90° to the object ball's path, along the tangent line, for any cut angle.
In more detail
It lets you predict cue-ball direction instantly: point your hand along the object ball's path and the cue ball goes at a right angle. It is also how you spot scratches into side and corner pockets before shooting.
People also ask
Does the 90° rule work with follow?
Not exactly — topspin bends the cue ball forward, so it ends up at less than 90°. For natural roll near half-ball, use the 30° rule.
Train it at the table
More guides on 90-degree rule
Cue-Ball Control & Spin
Stun Shot Pool Angle: Predict the Cue-Ball Route