Add together the number of HCP in the hand, and the number of cards in the two longest suits. If the resultant number is 20 or higher and most of the high cards are in the long suits, then an opening bid is suggested (the choice of which bid requires further analysis). As an example, a hand containing 11 HCP and 5–4–2–2 shape would qualify for an opening bid because the resultant number would be 20 (11 + 5 + 4) whereas 11 HCP and 4–4–3–2 shape would not (11 + 4 + 4 = 19). This method gives very similar results to length points as above except for a hand containing 11 HCP and 5–3–3–2 shape which gives 19 on the Rule of 20 (insufficient to open) but 12 total points by adding 1 length point to the 11 HCP (sufficient to open). Experience and further analysis are needed to decide which is appropriate.
For another discussion on the Rule of 20, please see Larry Cohen's article on this topic.
SOURCE: Wikipedia contributors. "Hand evaluation." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 23 Apr. 2013. Web. 24 Apr. 2013.