Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES
The 90° rule
Part II Breakup and Avoidance Shots
Note: 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
correspond to the numbers used on the website.
In my article last month, I introduced a very important principle of billiards called the 90° rule. It states that
when the cue ball strikes an object ball with no topspin or bottom spin the two balls will always separate at 90°,
regardless of the cut angle. Put another way, the cue ball will leave along the tangent line, which is perpendicular
to the impact line with the ball. Principle 1, NV 3.4, and TP 3.1 summarize and illustrate the important points (see
last months article for more information).
As shown in last month's article (Part I), the 90° rule is very useful for helping to prevent scratches. This
month (Part II), we look at how the same rule can also be used to plan break-up and avoidance shots. In next
month's article (Part III) we will look at how the 90° rule can be used to plan carom and billiards shots. Hopefully,
after this series of articles, you will appreciate how important this rule is, and will be able to use it in your game.
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 NV 3.4 and TP 3.1).