Mutually exclusive events are events in a common probability space that are pairwise disjoint. For 
such events,
 |
whenever 
,
where 
is the empty set. Consequently, the conditional
probability satisfies 
for 
whenever 
. When 
, the usual ratio definition of conditional
probability does not assign a value (Grinstead and Snell 2006, pp. 134-135).
