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General

X -
 

1.	The bidding symbol used to designate a double
2.	The card designation symbol indicating an unimportant card, irrelevant to bidding and play.

General

XX - The bidding symbol used to designate a redouble
 

General

X Factor - Positive interpersonal factors that differentiate top Bridge players, including: focus, killer instincts, and tenacity to never give up.  Also see Environmental Factors
 

Bidding

XYZ Convention - See Conventions