Classification
Bidding systems can be classified into two broad categories: natural systems and artificial systems. In natural systems, most bids (especially in the early phase of the bidding) denote length in the suit bid. In artificial systems, the bids are more highly codified, so that for example a bid of 1♣ may not be related to a holding in the club suit.
Natural system(s) are the "lingua franca" of bridge players, with regional variations. Thus, a new partnership can agree to play a natural system and understand each other fairly well. Players sometimes alter certain aspects of a system, adding their specific agreements or preferred conventions.
Structure and meaning of opening bids are the common determining factor for system classification: in most modern natural systems, opening bids of 1♣ through 2♣ have the same or similar meaning, with level-one bids denoting length in a suit. Artificial systems typically reserve at least one one-level suit opening bid for special purposes, unrelated to the suit.
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