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From Laws of Duplicate Bridge, Law #68:
B. Concession Defined
1. Any statement to the effect that a contestant will lose a specific number of tricks is a concession of those tricks; a claim of some number of tricks is a concession of the remainder, if any. A player concedes all the remaining tricks when he abandons his hand.
2. Regardless of B1 above, if a defender attempts to concede one or more tricks and his partner immediately objects, no concession has occurred.
Unauthorized information may exist, so the director should be summoned immediately. Play continues. Any card that has been exposed by a defender in these circumstances is not a penalty card, but Law 16d applies to information arising from its exposure and the information may not be used by the partner of the defender who has exposed it.
Law 70.A. should also be consulted:
Contested Claim or Concession
A. General Objective
In ruling on a contested claim or concession, the director adjudicates the result of the board as equitably as possible to both sides, but any doubtful point as to a claim shall be resolved against the claimer.
Law 71 may also apply when a side claims or concedes a trick that cannot have been won or lost through any normal* play of the cards:
A concession must stand, once made, except that within the correction period established under Law 79c the director shall cancel a concession:
1. If a player conceded a trick his side had, in fact, won; or
2. If a player has conceded a trick that could not be lost by any normal* play of the remaining cards. The board is rescored with such trick awarded to his side.
* For the purposes of Laws 70 and 71, “normal” includes play that would be careless or inferior for the class of player involved.
NOTE: Many clubs which hold games only once a week establish a time limit of the start of the next week’s meeting of that session as the end of the correction period for score corrections. This gives the players a full week to check for errors in the score, but it is too long a time to apply to canceling concessions because memories have had time to dim.