If they bury just one suit-showing bid, a double is a replacement for that suit. After such a replacement double, partner continues as if you had actually bid the suit.
If their bid doesn't bury any suit-showing bids, a double is a replacement for a bid of no-trumps at the lowest level that would have been possible without their bid. If that no-trump bid is still available, the double is a no-stop double, denying a stop in their suit, whereas the no-trump bid shows one.
| West | North | East | South | |
|---|
| | | 1 | 2 | |
| ? | | | | |
Assuming it is natural,
2 buried only spades, so the auction is not disturbed. As far as possible, all our bids mean the same thing. So:
| dbl | replacement; West would have responded 1 |
2 | natural, unlimited, denies four spades |
| 2 | limit raise |
| 2 | splinter agreeing hearts etc |
What if responder holds only clubs? With some hands, he might bid 3NT; otherwise, he should pass and look forward to partner re-opening the bidding with a takeout double.
| West | North | East | South | |
|---|
| | | 1 | 1 | |
| dbl | | | | |
South's
1 hasn't buried any suits, so double shows a hand that would have bid 1NT. As 1NT was still available, the double denies a heart stop, whereas
1NT would have confirmed one.
| West | North | East | South | |
|---|
| 1 | 1NT | dbl | 2 | |
| dbl | | | | |
East's double shows a hand that would have bid 1NT had North not got there first. West's double is a replacement for the buried
2.