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.

VIII
FORCED BIDS
AN opening bid is called "a free bid" because nothing has preceded to obligate the voluntary announcement that among other things its bidder has
at least two sure tricks' defense against any opposing
trump declaration. A "forced bid" is one made over
a previous declaration and does not claim ability to
win two defensive tricks. Probably but not necessarily, forced bid strength includes at least one sure
trick and the average help for partner of three
tricks. If lacking those two elements of strength
expected of an average hand the maker of the forced
bid should be able to safely continue his contracting
if overcalled by either partner or opponent, otherwise the forced bid is as useless as campaign
promises.
Of course any holding worth an opening bid of
one is equally worth a forced bid of like amount.
Any five-trick hand worth an opening bid may also
be bid two over another declaration, provided in
both instances that sufficient trumps are held to
spare those needed for ruffing leads of an opponent's suit.
The minimum requirements for a forced suit bid
are:
i. One sure trick;
2. Probable ability to win three trump tricks;
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