Here's another example from today's New York Post. West opened 1 club. (He had a 19 point 1NT hand, and was planning to re-bid 2NT.) North made a (takeout) double with (s) KQxxx (h) Qxxxx (d) xx (c) x, which according to partnership agreement, showed 5-5 in the majors. East passed, and South was forced to bid 1 H with (s) xxx (h) xxx (d) Kxx (c) xxxx. They went down three doubled vulnerable. East-West could have made game, but not slam in 3 NT or one of the minors.
The "rule of 20" says that you can OPEN when the sum of your HCP plus the length of your two longest suits is at least 20. That is, 10 HCP plus a 5-5 or 6-4 distribution, 11 HCP with a 6-3 or 5-4, 12 HCP with 5-3, and you need 13 HCP for a 4-3-3-3 distribution.
Here, I WOULD have doubled with (s) KQxxx (h) KQxxx (d) xx (c) x (ten high card points plus 5-5), but not with North's actual hand (only 17).
Would you find the "rule of 20" useful in this context (takeout double, or even "unusual" NT overcall showing two minor suits)? Would you refrain from doubling even with my "hypothetical" hand? Or would you have doubled like North with a tally of less than 20? Would vulnerability affect your application of this rule?