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.

XIV
BUSINESS DOUBLES
A DOUBLE means "business" (That the contract can be defeated): if not made at the first
opportunity afforded to double any declaration; or
if the contract doubled is either 2-No Trumps, 4 -odd in a major suit or 5-odd in a minor suit (Or
any lower bid agreed upon with partner).
The majority of players are not sufficiently quick
about doubling, allowing more than half their defeats of opposing bids to be undoubled. Unless
you fail to defeat about one in four or five contracts
that you double, you may rest assured that you do
not double often enough. Being quick to double
overbids slows up adverse contracting, but you cannot expect to defeat every contract that you double.
A "free double" occurs when a game contract has
been undertaken. A "forced double" is one made
of a contract for less than game. You may make
a free double upon probable ability to defeat the declarer a single trick, but you should not attempt a
forced double unless expecting to defeat the contract at least two tricks.
Frequently a double costs its maker a trick
through enabling the declarer to finesse through
located strength. So many close doubles fail to penalize that you should not shift your partner's make
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