Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES
The 90° rule
Part III Carom and Billiards Shots
Note: Supporting narrated video (NV) demonstrations, high-speed video (HSV) clips, and technical
proofs (TP) can be accessed and viewed online at billiards.colostate.edu. The reference numbers used
in the article (e.g., NV 3.4) help you locate the resources on the website.
In my previous two articles, I introduced the 90° rule. It states that when the cue ball strikes an object ball
with no topspin or bottom spin, the two balls will separate at 90°, regardless of the cut angle. The cue ball leaves
along the tangent line, which is perpendicular to the impact line (see Diagram 1). Principle 1 and NV 3.4
summarize and illustrate the important points of the rule. The 90° rule is very useful for helping to prevent
scratches (see Part I in the January, 2004 issue) and to plan break-up and avoidance shots (see Part II in the
February, 2004 issue). In this article (Part III) we will look at how the 90° rule can also be used to plan carom and
billiards shots. We will also see how the 90° rule is important in position play.
Principle 1 90° rule
With a stun shot where there is no topspin or bottom spin, after impact the cue ball will depart along the
tangent line, which is perpendicular (at a right angle) to the impact line. In other words the cue ball and
object ball paths, after impact, will be 90° apart (see Diagram 1, NV 3.4 and TP 3.1).