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From the Alert Procedures:
http://www.acbl.org/...Procedures.html
PART III: PRE-ALERTS
Pre-Alerts are designed to act as an early warning system of any unusual methods for which the opponents may need to prepare. Pre-Alerts must be given before the auction period begins on the first board of a round or match.
1) "TWO-SYSTEM" METHODS
Some pairs vary their system by position, by vulnerability, or a combination of the two. While this is legal, it is also something the opponents may need to know ahead of time. One example of this is agreeing to play a forcing-club system not vulnerable and "two over one" vulnerable.
Minor variations such as varying notrump range or jump overcall strength by vulnerability do not require a pre-Alert. These methods still require normal Announcements (notrump ranges; transfers) or Alerts (forcing Stayman over some notrump ranges) when appropriate.
As an aside, please note that it is not legal to vary your system during a session for subjective reasons, such as the skill level of the opponents which you happen to be playing at the time or which member of the partnership is making the call. You may, of course, alter your defenses in response to the opponents' methods.
2) SYSTEMS BASED ON VERY LIGHT OPENINGS OR OTHER HIGHLY AGGRESSIVE METHODS
If it is your partnership style to routinely open hands with fewer than 11 HCP, preempt with very weak (frequently worse than Qxxxxx) suits, and/or overcalls with fewer than 6 HCP at the one level, the opponents must be pre-Alerted.
3) SYSTEMS THAT MAY BE FUNDAMENTALLY UNFAMILIAR TO THE OPPONENTS
Players are expected to be prepared for the vast majority of systems that they may encounter at the bridge table. Common methods include either strong or weak notrumps with or without five-card majors. The forcing opening bid will most often be an artificial forcing opening of 1 or 2 .
When you play a system structured along different agreements than these, you should draw the opponents attention to your convention card before the round begins. In short, if you play a system that most players would not immediately recognize (such as a canapé system) or one the opponents may wish to discuss before the auction begins (a 10-12 1NT range with distributional requirements for minor-suit openings, for example), you are required to pre-Alert the opponents.
Peter