Summary: David Alciatore, PhD ("Dr. Dave") ILLUSTRATED PRINCIPLES
"90/30 Degree Rule Follow-up Part I"
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.
Since my book ("The Illustrated Principles of Pool and Billiards") came out, and since my
articles started appearing in BD, I have received many supportive comments, remarks, and
questions. That's one of the joys of being an author. Many of the questions have involved the 90
degree and 30 degree rules. Because of this, I have decided to write another series of articles
answering these questions for the benefit of all readers. In this article, I just want to answer a few
of the more basic questions and respond to some of the more general remarks. In future articles,
I'll look at the important effects of English and speed on the 90 and 30 degree rules. FYI, my first
set of articles dealing with the 90 and 30 degree rules appeared in the January through June,
2004 issues of BD. As with all of my past articles and resources, you can access them online at
For reference, Diagrams 1 and 2 summarize the 90 and 30 degree rules. Remember, the 90
degree rule applies only for a stun shot, where the cue ball is sliding at object ball impact, and the
30 degree rule applies only when the cue ball is rolling at object ball impact (see my July, 2004
article for more details). The 90 degree rule (see NV 3.4, NV 3.5, NV 3.6, and TP 3.1) states that