We have some interesting and unintentional linguistic doubles in English. How often have you heard someone say "two twins" or "an ATM machine"?
We are looking at interesting and intentional doubles in bridge. First, we are examining takeout doubles. When your partner makes a takeout double, what do you need to make a reply in no-trump?
The first thing to remember is that partner has implied shortage in the opener's suit. Consequently, you cannot rely on partner for any help there. So, if you bid no-trump, you should have a good holding in that suit. Second, the more you bid, the more points you promise. An advance of one no-trump shows 6-9 points, two no-trump 10-12, and three no-trump 13-15. With more, faint! And it is unlikely that you have four cards in an unbid major. Also, do not worry about being weak in another suit. Your partner has promised length there; it is his job to have that suit covered.
In this deal, South should leap straight to three no-trump. He has no interest in a major-suit contract and has clubs well under control.
After West leads the club jack, declarer sees eight top tricks: four hearts, one diamond and three clubs. The ninth winner will come from spades. South wins the first trick and immediately plays a spade to West's king.
If West persists with clubs, declarer will end with an overtrick. But if West shifts to the diamond king, South will win with dummy's ace and lead another spade. At the worst, he will lose two spades and two diamonds.