A direct summand of a group direct sum 
of additive Abelian
groups is either of the factors 
or 
. The map 
defined by 
is called the injection of the first summand, and the map 
defined by 
is called the projection onto the first summand. Similar maps 
are defined for the second summand 
.
The above definitions extend in a natural way to the group direct sums of more than two Abelian groups.
