Bidding system is set of agreements and conventions in bidding between two partners in contract bridge. A bidding system defines the meaning of every possible bid by each partner, and presents a codified language which allows the partners to find out about each other's holdings and determine the correct contract.

It should be noted that, by the rules of the game, the semantics of a bridge system must be public and known to the opponents. Thus, the opponents are entitled to know the meaning of every bid as defined by the system (but not the inferences about the partner's cards that a player draws by looking at his own hand). Also, they are entitled (when it's their turn to bid or play) to ask the partner of the bidder about the meaning of the bid during bidding phase.

Bidding systems can be classified into two large categories: natural systems and artificial systems. In natural systems, most bids (especially in early phase of the bidding) denote length and/or strength in the suit (cards) bid. In artificial systems, the bids are more highly codified, so that for example a bid of 1♣ may not be related to clubs at all.

Natural system(s) are a "lingua franca" of all the bridge players throughout the world (with certain regional variations). Thus, a one-time partnership can agree to play a natural system and understand each other fairly well.

Every regular bridge partnership often alters certain aspects of a system, adding their specific agreements or preferred conventions. Thus, most systems outlined below present guidelines or general frameworks rather than well-defined sets.

Classification

1. Natural systems in general have the following features:

• Level-1 suit bids denote at least 4 or 5 cards in a major suit, and 3 or 4 cards in a minor suit, with strength of about (11)12-20(22) high card points. The suit bid is generally the longest. The former criterion inflicts further classification into four-card major and five-card major systems.
• Bid of 1NT always presents a balanced hand in a narrow high card points range. The common ranges are 15-17 or 16-18 HCP ("strong notrump") and 12-14 ("weak notrump").
• Bid of 2♣ typically presents a very strong hand (23 HCP up).
• Bid of 2NT presents a strong balanced hand, usually 20-22 HCP.
• Meaning of bids 2♦, 2♥ and 2♠ varies. Two common approaches are that it shows either a weak two bid or an "intermediate" hand (20-22 HCP) with a long suit bid.

The most widespread natural systems are:

• Acol, featuring 4-card majors and weak notrump, originating in Great Britain
• Standard American, originally with 4-card majors but later transforming into 5-card majors.
• 2/1 game forcing, based on Standard American and gradually superseding it.

1. Artificial systems can be further clasified into:

1. Strong club systems are the most popular artificial systems, where opening of 1♣ shows a strong hand (typically 16 HCP up). Other level-1 bids are typically natural, but limited to about 15 points. The most popular strong club systems are:
   • Vanderbilt club (the predecessor)
   • Precision club
   • Blue club
2. In Small club systems, the bid of 1♣ is ambiguous, showing several types of hands. That typically includes some range of balanced hands, some hands with long club suit, and very strong hands. The represents are:
   • Vienna club (the predecessor)
   • Roman club, developed and used by famous Blue team
   • Polish club, originating (and standard) in Poland but also gained certain popularity worldwide
3. Strong diamond systems are similar to strong club systems, but the bid of 1♦ shows a strong opening, and the bid of 1♣ is typically ambiguous, as in small club systems. An example is Leghorn diamond, played by some top Italian pairs in 1970s.
4. Strong pass systems are highly artificial and fairly rare. In those systems, an initial pass shows a hand of opening strength (11+ HCP); as result, weaker hands must be opened with a bid instead (such bids are called "ferts", short for fertilizers). Strong pass systems are mostly banned by World Bridge Federation and other governing organizations from all competitions except the highest-level ones, because opponents cannot be reasonably expected to cope with such unusual approach.

• Relay systems are based on relay bids ? the artificial bids where one partner just bids the cheapest denomination (relay bid) and the other describes his distribution and high cards in detail (relay response) using a highly codified scheme. Such systems are out of the above classification (based on opening bid structure), as the relay feature takes place later in the auction. For example, relatively popular "Moscito system" has variants based on strong-club and strong-pass approaches. Symmetric relay is based on Precision club.