Correct contract bridge / by E.V. Shepard ; including the official Laws of contract bridge adopted by the Whist Club, New York, and by all the leading clubs in America.

70 CORRECT CONTRACT BRIDGE
that its bidder holds at least 6, more probably 7,
honor tricks out of a probable total of 15, leaving
not more than 8 or 9 among the three other players.
Except in an emergency the doubler should at least
be as strong as the player whom he doubles; if this is
true in your case, what can your partner be expected
to hold in honors? Probably he will have great difficulty in fulfilling his contract, let alone going game.
That is why a double of 2-No Trumps must be considered as meaning "business."
In making informatory doubles the vital factor is
furnishing your partner sufficient aid to probably
enable him to fulfill his contract, the number of
sure tricks held is of secondary importance. Largely
because the majority of Auction players cannot
value a hand closely, rules for doubling in the older
game stressed the number of sure tricks required,
taking care to insist upon enough sure tricks to ordinarily give sufficient assisting tricks as well, although there is no necessary relationship between
defensive and offensive values. In Contract Bridge,
if you will note that no informatory double is attempted without holding at least 2 sure tricks and
also the necessary number of assisting tricks, on all
the informatory doubles yet mentioned you need
not worry about the precise number of sure tricks
required for this and for that particular double;
the number of sure tricks beyond two will take care
of itself.
One bid frequently encountered in Contract"the slam signal," either as an opening bid of 2-odd
or as a jump by the opening bidder's partner from