A Priori Probabilities  A priori comes from the Latin phrase
meaning "from the former", a selfevident proposition. Relating to
playing a hand of Bridge, after seeing one hand and dummy (26 cards), a
player's
initial assessment of card distribution would be "a priori". However,
as the cards are played and information about suit breakage was observed,
players would revise their probability assessment (a posteriori). The
a priori probability of two hidden hands
holding a certain number of cards is based on mathematical odds.
Aspiring Bridge players make mental references to the distribution when
determining the best line of play. An a priori example showing initial
probabilities relating to aggregate suit holdings when partner opens 1 Notrump (balanced) would be:
Length of fit 
3 
4 
5 
6 
7 
8 
9 
10 
Percentage 
1 
6 
17 
27 
26 
15 
5 
1 

Generally, when opponents hold an
even number of cards, the number of cards held will not
break evenly between the opponents. When opponents hold an odd number
of cards, the number of cards held will break evenly. See
Probability of Distribution Table 