~Voluntary Bid of 5 of a Major

For the bid to have been voluntary, it must have been possible to bid four of the major. Since there is no bonus for bidding five of a major, the bid is a slam try (unless raising partner’s preempt in a competitive auction. This is commonly recognized as an “advance sacrifice.”

If the opponent’s have not bid, partner would use the Grand Slam Force if only concerned about trumps. Therefore, partner would never bid 5H or 5S holding the three side-suit aces and only concerned about the trump suit.

If the enemy has bid a suit, 5 of the major shows concern about losing two tricks in the enemy suit and asks partner to bid six of the agreed trump suit if holding second-round control of the enemy suit. Second-round control could be a guarded king, the KQ of the enemy suit, or a singleton in 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.

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: because the partnership has available the Grand Slam Force bid of 5NT to try for a grand slam if the trump suit is solid, the voluntary bid of 5 of a major is never a Grand Slam Forcing bid.