I couldn't put this rule any better than my good friend, Marty Bergen, so I am using his writing on the topic (as below). Of course, I have his permission to show it here--he's proud of the rule.
What do I (Larry) think of the Rule of 20? My opinions at the end in BLUE.
The Rule of 20 (by Marty Bergen)
After teaching bridge for over 20 years, I thought I had seen it all. However, I had