The Rule of Nine is a concept employed by the responder after a certain bidding situation, which is described below. An example should clarify the situational element.
North
East South West 1 2 Pass Pass ?
The question for North, the opener, is whether to reopen the bidding with a double. The situation is that North has opened the auction, the left hand opponent, East, has made an overcall, which is then followed by two passes. There could be two reasons why South passed:
1. South has no values. 2. South has values and wanted to make a Penalty Double, but could not since a double would have been interpreted as meaning the two unbid suits.
In determining the correct reason as to why South passed, the inference made by North is that South has values if North is short in the suit of the opponent, two or less. In the case that North holds three or more cards in the suit of the opponent, then the inference is that South has no values, with which to bid.
Therefore, North should follow the guidelines below when deciding to reopen the auction with a double:
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