It is frequently most difficult for a bidder to determine whether he is justified in advancing his own or his partner's declaration, and when in doubt it is generally better to err on the side of conservatism.
The continuation of a No-trump without the adverse suit thoroughly guarded is most dangerous, and should be risked only when the Declarer is convinced beyond doubt that his holding justifies it, or when the partner has shown that he can stop the threatening suit.
When the partner, either as Dealer or Second Hand, has declared one No-trump, the bid has unquestionably been based upon the expectation of average assistance, and unless able to furnish more, a higher call should not be made. If, however, the partner bid twice, without aid, two tricks unquestionably justifies assisting once.
The minimum trick-taking ability with which an original suit declaration is made being appreciably greater than the number of tricks contained in a border-line No-trumper, the former should be assisted with less strength than is required to advance the latter. With two sure tricks the partner's suit call should be helped once by a player who has not declared, but whether a No-trump should be aided with just two tricks and no chance of more is a question depending upon the judgment of the bidder and upon whether one of the tricks is in the adverse suit. With two sure high-card tricks and a five-card suit, but without the adverse suit guarded, the five-card suit is generally the call, especially if two in it will be sufficient. Three Clubs, however, should not be declared without due consideration, as that declaration is recognized as demanding two No-trumps from the partner if he have the adverse suit stopped.
Being void or holding only a singleton of a suit, especially if it be the suit declared by the adversary, is to be considered in reckoning the trick-taking value of a hand which contemplates assisting a partner's Trump declaration. For example, four small Hearts, the Ace and three other Clubs, and five small Diamonds, when the partner has called one Heart, are worth three or four tricks, although the hand contains but one Ace and no face card. Holding such a combination, a partner's bid of one Heart should be advanced at least twice.