Given the direct sum of additive Abelian groups 
, 
and 
are called direct summands. 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 direct sums of more than two Abelian groups.
