David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES "90 and 30 Rule Review" Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES "90º and 30º Rule Review" Note: Supporting narrated video (NV) demonstrations, high-speed video (HSV) clips, and technical proofs (TP), and all of my past articles, can be accessed and viewed online at billiards.colostate.edu. The reference numbers used in the article help you locate the resources on the website. If you have a slow or inconvenient Internet connection, you might want to view the resources from a CD-ROM or DVD. Details can be found online at: dr-dave-billiards.com. Periodically, I will write an article presenting a concise summary of principles from past articles. I think it is useful to review and reflect occasionally to help keep things in perspective and to reinforce important information. I'm a teacher in my other life and I know how important repetition and reinforcement are to learning. This article summarizes my previous six articles dealing with the 90° and 30° rules. Hopefully, by seeing all of the results in one place, knowing when and how to apply each rule will become clearer. In my January, 2004 article, 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 my January, 2004 article), plan break-up and avoidance shots (see my February, 2004 article), plan carom shots (see my Collections: Engineering