In every auction, one partner or the other (not both) needs to know the total value of the partnership's combined holding as well as the hands' important features. We need to know these in order to place the final contract correctly.*¹ All the features one can know about can be boiled down into four topics: hand pattern; point count*²; defensive tricks; and controls.*³

By definition, The Captain is the partner of the player who first limits his hand. Limiting means announcing a hand whose value is known to within one trick, usually a three-point range.*⁴ Defining a captain in this way simplifies the bidding process and allows the Captain's hand to remain partly unknown, hindering the defenders as much as possible.

Captaincy Rules

• The Captain is the partner of the player who first limits his hand.
• Once partner becomes Captain, Captain's partner may not take charge of the auction; he merely follows orders.
• The instant Captain knows the limitation and scope of the combined hands, Captain is obligated to do one of two things:
  
  1. Bid the final contract without further ado, or
  2. Instruct partner to bid on, clarifying some feature of an already limited hand - a feature on which the contract depends.*⁵ There are no extra bids.
  ,

In the example hand, North's 2NT call limits his hand to a two-point range and denies holding four cards in the heart suit. This limits his hand and makes South the Captain.

Captain South was understandably confused when North took it upon himself to bid on after South had placed the contract.

North's correct call (3 instead of 2NT) reveals the heart fit and limits his hand (16 to 18), allowing South, whose hand at that time is still unlimited, to make an informed decision about the partnership's optimum spot.

*¹ Which can include bidding what we know we can make, doubling the opposition for penalty, letting them have it undoubled, or allowing ourselves to be pushed to a higher level.
*² This can be either high card points or Responder Count.
*³ Since this is neither a contested auction nor a slam try, we'll discuss defensive tricks and controls another time.
*⁴ Modern bidding has defined some two- and four-point ranges (like North's rebids in this example), nevertheless maintaining the concept of the one-trick range limitation.
*⁵ Examples: partner being at the top or bottom of a known range; holding some number of aces, or a particular ace; holding a notrump stopper, etc.