In natural bidding system

Bidding system

A bidding system in contract bridge is the set of agreements and understandings assigned to calls and sequences of calls used by a partnership, and includes a full description of the meaning of each treatment and convention...

s most notrump (NT) bids are made with balanced hands and within a narrowly defined high card point (HCP) range. In these systems, such as Acol

Acol

Acol is the bridge bidding system that, according to The Official Encyclopedia of Bridge, is "standard in British tournament play and widely used in other parts of the world". It is named after the Acol Bridge Club, previously located on Acol Road in London NW6, where the system started to evolve...

 and Standard American

Standard American

Standard American is a common bidding system for the game of bridge in the United States, also widely used in the rest of the world. This system, or a slight variant, is learned first by most beginners in the U.S. and may be referred to as 'Goren'; a dominant version used in on-line computer...

, NT bids are limit bids and therefore are not forcing. Bearing in mind the need to bid only to the optimum contract and no higher, bids above game are made only in specific circumstances, one of which is to alert partner to the fact that a slam may be possible and inviting partner to take part in the decision making process.

• Before looking at the detail, it is necessary to understand that bridge theory and practice suggest that the HCP method of hand evaluation
  Hand evaluation
  In contract bridge, various bidding systems have been devised to enable partners to describe their hands to each other so that they may reach the optimum contract. Key to this process is that players evaluate and re-evaluate the trick-taking potential of their hands as the auction proceeds and...
  
  , together with common sense concerning balance and cover in all suits, is the best for deciding the level of NT contracts, thus:
  • 25+ HCP is sufficient for a game 3NT
  • 33+ HCP should yield 12 tricks
  • 37+ HCP will probably produce a grand slam

• Assuming a weak NT bidding system
  Bidding system
  A bidding system in contract bridge is the set of agreements and understandings assigned to calls and sequences of calls used by a partnership, and includes a full description of the meaning of each treatment and convention...
  
  , for example Acol
  Acol
  Acol is the bridge bidding system that, according to The Official Encyclopedia of Bridge, is "standard in British tournament play and widely used in other parts of the world". It is named after the Acol Bridge Club, previously located on Acol Road in London NW6, where the system started to evolve...
  
  , this is how quantitative bids work:
  • An opening bid of 1NT shows 12, 13 or 14 HCP.
    • If responder has 21 HCP, then a small slam looks certain (21 + 12 opener's minimum = 33) and should be bid.
    • If responder has 18 HCP or less, then even a small slam is not possible (18 + 14 opener's maximum = no more than 32)
    • If responder has 19 or 20 HCP, then a small slam is a possibility but more information is needed about opener's hand before it should be bid. This is where a quantitative bid should be made:
      • a bid of 4NT "invites" opener to bid 6NT with a maximum holding of 14 HCP (19 + 14 = 33 which is sufficient)
      • to pass with a minimum 12 HCP (20+ 12 = only 32)
      • with partnership agreement, to bid 5NT holding 13 HCP - asking partner to bid 6NT with 20 HCP and to pass holding 19 HCP.
  • An opening bid of 2NT shows 20, 21 or 22 HCP.
    • If responder has 13 HCP, then a small slam looks certain (13 + 20 opener's minimum = 33) and should be bid
    • If responder has 11 or 12 HCP, then a small slam is a possibility but more information is needed about opener's hand before it should be bid. This is where a quantitative bid should be made:
      • a bid of 4NT "invites" opener to bid 6NT with a maximum holding of 22 HCP (11 + 22 = 33 which is sufficient)
      • to pass with a minimum 12 HCP (12+ 20 = only 32)
      • with partnership agreement, to bid 5NT holding 21 HCP - asking partner to bid 6NT with 12 HCP and to pass holding 11 HCP.
  • When responder is even stronger and is considering whether a small or grand slam is better (and only these two options), then the initiating bid is 5NT (not 4NT)
• Similar bids can be made using a strong no trump bidding system, for example Standard American
  Standard American
  Standard American is a common bidding system for the game of bridge in the United States, also widely used in the rest of the world. This system, or a slight variant, is learned first by most beginners in the U.S. and may be referred to as 'Goren'; a dominant version used in on-line computer...
  
  , by adjusting the HCP count accordingly