Bidding in depth
Open a hand that is short of the usual 12 points when its high-card points plus the lengths of its two longest suits reach 20.
An opening bid normally needs about 12 points. Some hands a point short are still worth opening, because long suits win tricks that high cards alone do not measure: two five-card suits will usually produce more tricks than a flat 12-count. The Rule of 20 is a quick test for which of those light hands to open.
Add your high-card points to the lengths of your two longest suits. Open when the total is 20 or more.
♠ A K 9 7 4 ♥ K J 8 6 3 ♦ 8 ♣ 7 4 has 11 high-card points and two five-card suits: 11 + 10 = 21, so open 1♠.
♠ K Q 7 4 ♥ A J 6 2 ♦ J 8 3 ♣ 9 5 has the same 11 high-card points, but its two longest suits are four-card suits: 11 + 8 = 19, so pass.
Our sources set a floor of 10 high-card points: below that, do not open on the rule however long the suits are. Seat and vulnerability count too. Opening light is at its safest in third seat, where partner has already passed and a pre-emptive effect is worth having, and at its riskiest when you are vulnerable and the opponents are not.
BidMaestro does not apply the rule directly. It counts high-card points plus one point for each card over four in a suit, takes one point off a completely flat 4-3-3-3 hand, and opens at 12. Every hand that satisfies the Rule of 20 reaches 12 by that count as well, so the rule never opens a hand the app would pass. The app opens a few hands the rule declines, most often 11 high-card points with a five-card suit.
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