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).
In the game of contract bridge, a convention is an agreed-upon meaning for a call (a bid, double or redouble, or a pass) during the auction phase of the hand. Often, the inventor of the convention gives it a name; some widespread conventions got a name after their (perceived) authors.
Conventional opening leads and discards may also be used. The term, however, usually denotes just a bidding convention.
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.
