Some Methods Used to Indicate Doubles (or Lack of) in 42
Commonly used and accepted legal methods:
1. Sluffing double to indicate second highest domino held.
2. Sluffing domino whose high end indicates double held.
Methods requiring pre-game understandings:
3. Sluffing domino whose low end indicates double held.
4. Sluffing domino whose high end indicates double NOT held.
5. Sluffing domino both ends indicate doubles NOT held.
Some partners don't indicate at all, adding a sixth variable to the mix.
The first method is deducible. One would not normally sluff a double unless he also holds the next highest domino in that suit.
The second method is commonly practiced, but false indications can occur with unexpected results.
Polling indicates the last three methods are considered unfair play because they require pre-game "understandings" between partners.
Paul Proft, E-mail
Note: As of 9 Jan 2018, the N42PA rules for sanctioned 42 tournaments state the following:
Suggestion: Preamble to organized tournament rules.
17. Any physical signals or cheating of any kind will not be tolerated. Examples include tapping dominoes on the table before being played, pointing dominoes in certain directions intentionally as a strategy, encouraging your partner to bid or pass during the bidding cycle, etc. Also bids meaning specific information will not be tolerated. An example would be if a team agrees that every time they bid 30, they always have the double 5, or they always have at least 3 doubles, etc. An agreement for a bid to mean non-specific help is permissible (but frowned upon by some players).
(Social players in non-tournament games may play by whatever rules they agree on, e.g., variations, etc.)