What is the 90° rule?
The 90° rule states that for a stun shot, where the CB has no top or bottom spin at impact with the OB, the CB and OB separate at 90°, regardless of the cut angle (except for a straight-in shot, in which case the CB stops in place). In other words, with a stun shot, the CB heads and persists along the tangent line direction, which is perpendicular to the line of centers (the line between the center of the ghost ball and OB).
The following diagrams illustrate the 90° separation angle between the CB and OB and how the tangent-line direction relates to other important CB position control reference lines.
Here’s a convenient 1-page summary resource page summarizing all of the important points of the 90° rule, and here’s a video demonstration:
Here are some other relevant video demonstrations:
- NV 3.4 – 90° rule with various cut angles
- NV 3.5 – Using your hand to visualize the 90° rule impact and tangent lines
- NV 3.6 – Stop shot
- NV 3.7 – Using the 90° rule to check for and prevent a scratch
- NV 4.1 – Stun shot
Here’s a good drill for practicing stun shots: stop/follow/draw drill.
Concerning friction and english effects, see the following instructional articles:
- “90° and 30° Rule Follow-up – Part II: speed effects” (BD, March, 2005)
- “90° and 30° Rule Follow-up – Part III: inelasticity and friction effects” (BD, April, 2005).
- “90° and 30° Rule Follow-up – Part IV: english effects” (BD, May, 2005).
- “90° and 30° Rule Follow-up – Part V: the final chapter” (BD, June, 2005).