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 the following experience in the winter of 1994, and it made a lasting impression on me. My class consisted of 28 experienced players, and I will never forget that first hand. The dealer held:

K Q 5 4
A 8 7 3
6
K 10 6 4

I was amazed as player after player passed this hand. Only one person opened. What was going on? I immediately stopped their non-bidding and asked: “Do you open with 13 points?” Everyone answered yes. (Whew!) I now asked the $64,000 question. “How do you count your points when you pick up your cards?” Seventeen students answered that they simply counted their high card points (HCP) and added points for distribution only if they found a fit. Ten students answered that they added points for length to their HCP: one point for a five-card suit, two points for a six-card suit, etc. I am familiar with this technique, but I cannot agree with any method of evaluation that calls for passing hands like this one.

The one student who opened said that she had added “short-suit points” to her HCP. One point for a doubleton, two for a singleton, and three for a void. With 12 HCP plus two points for the singleton, she was happy to open. This was the technique I learned when I took up bridge. How did I resolve the confusion? I taught them The Rule of 20.

In first and second seat, add the length of your two longest suits to your HCP. When the total is 20 or more, open the bidding. With less, do not open at the one level. Here is how it works. It is a matter of simple addition: HCP+ # cards in longest suit+ # cards in second longest suit. This is all you must know to determine whether you should open the bidding in first or second position (i.e., when partner has not had a chance to pass). If there is a tie for longest or second-longest, you can select either; I always use a major suit for my computation.

Try some examples.

The first is the hand that only one player opened in class.

K Q 5 4
A 8 7 3
6
K10 6 4

4 spades & 4 hearts + 12 HCP = 20. Open 1.

A Q J 8 6 5
--
9 7 2
K 7 5 4

6 spades & 4 clubs + 10 HCP = 20. Open 1.

K J 5
A 8 7 5
Q 7 5
Q 6 2

4 hearts & 3 spades + 12 HCP = 19. Pass.

8 7
Q 5 4
A K Q 9 7 6 4
9


7 diamonds & 3 hearts + 11 HCP = 21. Open 1.

The purpose of counting points is to evaluate your trick-taking potential to bid to the correct contract. However, you cannot accurately assess your values if you count only HCP. The reality of bridge life is that hands with long suits and short suits have far more potential than their balanced counterparts. Give The Rule of 20 a chance. There is no question that The Rule of 20 will increase your chances of having an opening bid. Is this desirable? Here are my thoughts:1) You bid more accurately after your side opens. 2) It is much easier to open than to overcall. 3) It must be right to get in the first punch. I hate to guess after my opponents have bid, particularly if they have preempted. 4) It is more fun to bid — absolutely, positively. If passing all afternoon is your idea of a good time, I suggest you check your pulse.

Larry's thoughts:
Thank you Marty for giving permission to print your Rule. Now for my thoughts. I think the Rule of 20, is a good guideline, especially for newer players. It gives an immediate (and easy) ballpark estimate of what is or isn't an opening bid. However (and Marty would be the first to agree), it is just "general advice." It is not to be followed religiously. There are many tiny outside factors which need to be considered, such as:
1) Spot cards (especially 10's and 9's).
2) Vulnerability (if borderline, be more aggressive if not vul).
3) Points in long suits (A Q 10 7 5
A 10 9 6 5
4 3
2
is much better than 8 6 5 4 2
9 7 6 5 4
A Q
A.
4) Short honors should be discounted a bit (such as singleton kings or doubleton queens or jacks).
5) Suits such as AJ10 or AQ10 are worth more than their point count, especially if accompanied by length.

Some writers have carelessly debunked the Rule of 20. What they really mean to say is that the Rule is a good basic starting point, but can use some fine-tuning.

Link to entire Bergen article.