Conventions in depth
A defence to the opponents’ 1NT in which the double is for penalty and the two-level overcalls show one- and two-suited hands.
A natural penalty double is fine when you are strong, but most of the time you would rather contest the deal on distribution. Cappelletti (also called the Hamilton convention) gives the low overcalls artificial, shape-showing meanings.
Over the 2♣ one-suiter, partner relays with 2♦ to hear the suit (or, holding a good six-card club suit, passes 2♣). The overcaller then shows the long suit, pass or correct, passing 2♦ with diamonds or correcting to 2♥ or 2♠ with that major. With clubs the overcaller corrects to 3♣, because 2♣ cannot be bid again.
Over the two-suiters, partner chooses the fit. Over 2♦ (both majors) partner picks the better major. Over 2♥ or 2♠ (that major plus a minor) partner has four choices. With three or more cards in the major partner passes or raises: to three of the major as an invitation with about 11–12 points counting distribution, or to game with about 13 or more. With fewer than three partner bids 3♣, pass-or-correct: the overcaller passes with clubs or corrects to 3♦ with diamonds, and the hand plays in the minor. A balanced hand of 11 or more high-card points with no fit may instead bid a natural, invitational 2NT, or 3NT outright with 17 or more. Over 2NT the overcaller passes with a minimum, bids 3NT with about 14 or more, or retreats to the minor with a shapely minimum.
Throughout, the aim is to find an eight-card fit and take the auction away from the strong hand.
This is the treatment taught by Joan Butts, which BidMaestro follows. The original Cappelletti uses 2NT itself as the ask for the minor, and some modern versions add a stronger 2NT relay with coded replies. BidMaestro plays neither: the minor is found with 3♣ pass-or-correct, and 2NT is a plain invitation.
