The Laws themselves say nothing about what systems should or shouldn't be played. They do, however, limit your choice of system in two indirect ways:
1. Concealed partnership understandings are prohibited. Whatever system you choose to play, you must be willing and able to explain how it works to your opponents. No secret codes like pitchers and catchers use in baseball. This also means that, as you play with a partner over a period of time and learn more about his bidding style, you lose the ability to hide behind "we haven't talked about it" as an excuse for not being able to explain what your partnership's bidding means. "Partnership understandings" can emerge from remembering how your partner has bid in the past and choosing your own calls to accommodate his tendencies, just as easily as they can from a conversation where you agree to play a certain system.
2. Sponsoring organizations may regulate use of conventions. You and your partner can choose to make whatever agreements you wish about how you use natural bids. No-one can tell you whether to play a 10-12 1NT or a 15-17 1NT, whether your 2
opening should promise five cards or six (or seven!). What each playing site can do is tell you whether you're allowed to use the Multi 2
, or Bergen Two-Under Preempts, or even Stayman and Takeout Doubles. (Yes, between 1913 and 1920 there really was a major conflict over whether or not takeout doubles should be allowed!)
Here is the definition of a convention, taken straight out of the law book:
A call that, by partnership agreement, conveys a meaning other than willingness to play in the denomination named (or in the last denomination named), or high-card strength or length (three cards or more) there. However, an agreement as to overall strength does not make a call a convention.
Any bid that doesn't meet that definition, is entirely up to the players to choose whether to use. Any bid that does, the sponsoring organization gets to tell you if it's permitted or not. The regulations set up by most major bridge organizations for their events, including the World Bridge Federation and the ACBL, are posted online.
The policy at Swan Games and many other online bridge clubs is to allow any system, but require unusual systems to be announced in advance and suggested defences for the very unusual systems to be provided.
Nothing. The Laws simply require that you disclose information about your system "in accordance with the regulations of the sponsoring organization."
Almost all face-to-face bridge tournaments, and most online sites, require you to fill out a convention card describing your system in advannce, and to call your opponents' attention to any unusual agreements you have about a particular bid as soon as you make that bid.
As with systems policy, most major bridge organizations have their rules about alerts posted on the web. Here are links to the WBF and ACBL regulations.
The laws require you to give your opponents proper information about your partnership agreements. Players often confuse two very different situations, which are treated in opposite ways by the laws:
If you mis-explain your agreements to your opponents, it's important that they receive the correct information as soon as possible: the sooner they find out, the less damage is likely to have been done to them. If it turns out that your opponents would have bid or played differently and gotten a better score had they been told promptly what your system was, the director will adjust the score. They are entitled to the best score that was likely had you correctly explained your bidding (or the score they actually got at the table, whichever is better.) Your score correspondingly may be reduced, if it's possible you would have gotten a worse result had the opponents had the correct information.
Often there is room for judgment and debate as to what would have happened in an ideal world where the opponents had correct information. At the table the director will take his best guess at what he thinks should have happened, giving the benefit of the doubt to the innocent victims. The most common function of an appeals committee is to take more time to study the cards and work out in detail what might have happened, and make sure that the fairest score is assigned.
If the hand you hold doesn't match the the meaning of the bid you have made, there is usually no adjustment. There is no law against temporarily forgetting your own system -- it happens to everyone! -- and there is a law guaranteeing you the right to deliberately deviate from your system occasionally (see the section on psyching.)
You can't go back and change your bid (see the section on misclicks) but you ARE allowed to take your best shot at finding a playable contract. As long as your opponents are correctly informed of your system and your partner doesn't receive any unauthorized information from you telling him what is going on, you keep the score you earn at the table after a misbid, however good or bad it is. If there is a score adjustment, it will be because of the misinformation or unauthorized information, not because of the misbid itself.
The rules about systems are found in Laws 40, 73, and 75.