Voluntary Bid of 5 of a Major - folksbridge21/system2 GitHub Wiki

5 of a major bid is voluntary if four of the major bid is still possible. Voluntary five of a major a slam try (unless it is raising partner’s pre-empt in a competitive auction. This is commonly recognized as an “advance sacrifice.”)

If the enemy has bid a suit, 5 of the major shows concern about losing two tricks in the enemy suit. It asks partner to bid six of the major suit if holding second-round control of the enemy suit. If the partnership has bid three suits, a bid of 5 of the trump suit asks about second-round control of the unbid suit. Second-round control could be a guarded king when on side, KQ or a singleton in the enemy suit. Responses

Pass with no control. Six of the agreed trump suit shows second-round control of the suit in question. Any suit bid other than the trump suit shows first round control of the suit in question as well as the cue-bid suit. Note: The voluntary bid of 5 of a major is never a Grand Slam Forcing bid. The Grand Slam Force bid of 5NT is available for the partnership to use for a grand slam try if the trump suit is solid.