Summary: David Alciatore, PhD ("Dr. Dave") ILLUSTRATED PRINCIPLES
"90° and 30° Rule Follow-up Part V: The Final Chapter"
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.
You might also want to view the resources from a CD-ROM. See the website for more details.
Last month, I lied! I wrote that I was done with my series of articles on the 90° and 30° rules,
and I would move on. Well ... I couldn't! These rules are so important and so often
misunderstood (or even often unknown, as is often the case with the 30° rule), I felt like I needed
wrap up with one last article on the topic. I wanted to bring together the two rules and show
where they overlap. I promise that this REALLY is the last article ("The Final Chapter") in the
If you don't remember what the 90° and 30° rules are and when they apply, see NV 3.4-3.5
and NV 3.7-3.10. Readers with engineering or physics backgrounds might also find TP 3.1 and
TP 3.3 interesting. Remember, the 90° rule states that for a stun shot, where the cue ball is
sliding at object ball impact, the cue ball and object ball paths separate at 90° (i.e., the separating
paths are perpendicular). The 30° rule states that when the cue ball is rolling when it hits the
object ball, and when the cut angle is between a 1/4-ball and 3/4-ball hit, the cue ball's path will
be deflected by approximately 30°. If these previous two sentences are not clear, you might want
to look at the online videos and past articles on my website.