Bridge is not a game of secret messages. Opponents are entitled to know just as much about your system as you do.
Alerting is a warning to them that your partner's bid or pass should not be taken at face value. Doubles, redoubles, 2C in the specific auction 1NT (Pass) 2C, bids above 3NT and cue bids of a suit bid or shown by the opponents are considered to be self-alerting as they ring their own warning bells and must not be alerted.
Saying "Alert" is the quickest way to stop next opponent from acting without the special information. Circling the call is confirmation that the call has been alerted.
Alerting is done immediately the call is made. If your RHO has called before your alert then Law 21 comes into play as a failure to alert as required by the Regulating Authority is deemed to be misinformation.
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Article Posted: August 19, 2009