In the partnership card game contract bridge, the San Francisco convention is a slam bidding convention, or a special usage by partnership agreement in order to help choose among the five, six, and seven levels for the final contract. One partner bids 4NT to ask, and the other replies, in code, to show aces and kings in one bid. Thus it is an alternative to the Blackwood convention family.
The San Francisco reply is usually sufficient for partner to infer the precise numbers of aces and kings in the replying hand, hence in the two partnership hands. So it is useful where partner can choose the contract level by knowing the numbers of aces and kings that are "missing" in the two opposing hands.
The San Francisco scale is three points for an ace, one for a king. The asking bid is 4NT with these replies:
Except for 5♣=0-2, 5♠=5, and 6♦=8, the replies are equivocal with reference to the entire 52-card pack, but the other replies may be unequivocal given reference to another hand, typically partner's hand. For example, if partner holds two kings then 5NT must be two aces.
The San Francisco replies take so much space that a weak or medium-strength hand may need to investigate slam by other methods.
As slam bidding conventions at or near the 4NT level, San Francisco and Norman four notrump have been superseded by the Blackwood convention and its variants, which show at once only the number of aces or keycards, and show kings subsequently if at all.