Rule of Ten or Rule of 10
This mathematical concept originated back in the days of the game of Whist, and to which no name can be attached. It is similar to their cousin concepts:
1. the Rule of Eleven, which may determine the play of a card by partner, who knows that the lead card is the fourth highest card down as per partnership agreement and 2. the Rule of Twelve, which may determine the play of a card by partner, who knows that the lead card is the third highest card down as per partnership agreement.
The Rule of Ten states that the lead is the fifth highest card and that the partner subtract the number of the card from the number 10, and the result obtained is the number of higher cards than the one led in all of the other three hands.
The following illustration should clarify this concept. The contract is 4, the declarer is South, and the lead is 3:
3 of Hearts
AK98 Q86 QJ53 J4
J532 K7543 K10 107
10 J109 842 A96532
Q764 A2 A976 KQ8
The partner of West will know immediately, per partnership agreement, that the lead is the fifth best and play accordingly, knowing that the declarer only has one card higher than the 3 in the Heart suit. This information is arrived at by subtracting the lead, the 3 from the number 10, which equals 7, and therefore seven cards higher than the 3. Three higher cards are visible in the dummy, and West can see three higher cards in his holding. Therefore, the declarer only has one higher card in his holding.
If you wish to include this feature, or any other feature, of the game of bridge in your partnership agreement, then please make certain that the concept is understood by both partners. Be aware whether or not the feature is alertable or not and whether an announcement should or must be made. Check with the governing body and/or the bridge district and/or the bridge unit prior to the game to establish the guidelines applied. Please include the particular feature on your convention card in order that your opponents are also aware of this feature during the bidding process, since this information must be made known to them according to the Laws of Duplicate Contract Bridge. We do not always include the procedure regarding Alerts and/or Announcements, since these regulations are changed and revised during time by the governing body. It is our intention only to present the information as concisely and as accurately as possible.
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