A module  over a unit ring 
 is called faithfully flat if the tensor
 product functor 
 is exact and faithful.
A faithfully flat module is always flat and faithful, but the converse does not hold in general. For example,  is a faithful and flat 
-module, but it is not faithfully flat: in fact 
 reduces all the quotient modules 
 (and the maps between them) to zero, since for all 
 and all 
: